One of the most infamous magical weapons in Dungeons & Dragons, Blackrazor is one of the three artifact-tier magical weapons that first appeared in the AD&D 1e module White Plume Mountain, alongside Wave and Whelm.
Blackrazor is essentially a rip off of Stormbringer, the soul-eating demon-blade wielded by Elric of Melnibone in Moorcock's dark fantasy Sword & Sorcery novel series. As such, Blackrazor takes the form of a greatsword with a jet-black blade, sometimes described as being filled with star-like glimmers of light. A physical embodiment of nothingness, sometimes stated as having been forged from a dying universe or something along those lines, Blackrazor is a soul-eating blade driven by an incessant hunger for the lives of others, and which compels its bearer to hunt and slay people. This traditionally gives it incredible lethality in battle, and makes it a handy way to circumvent certain unkillable enemies, but it also traditionally has a drawback - namely, it tends to react poorly to attempts to use it on constructs or the undead, usually resulting in the bearer taking damage instead of the target, as its unsated hunger causes it to gnaw at them instead.
Blackrazor reappeared in 3rd edition as a Legacy Weapon, a kind of "evolving" magical weapon. It then reappeared as an artifact weapon in 4th edition. Finally, in 5th edition, it appeared alongside Wave and Whelm again in the Dungeon Master's Guide section on sentient magic items, and was mentioned as one example "patron" for the Hexblade patron for warlocks.
Having appeared in all five editions of D&D so far, Blackrazor has remained surprisingly consistent, given the different rulesets it's come under.
As mentioned, Blackrazor first appeared in 1e's "S2: White Plume Mountain" module. Here, it was a sentient +3 sword with Int 17 and Ego 16. Its sole purpose was to consume souls; if you refused to feed it, it grew restless, with its Ego growing by +1 point for every 3 days of "starvation" until it was strong enough to take you over and kill a sapient humanoid, whereupon it went back to normal. When used to deliver a killing stroke, Blackrazor ate the victim's soul; not only did this prevent them from ever being resurrected, but it gave the bearer a number of bonus levels equal to the victim's level and it gave them hitpoints equal to the victim's total hitpoints. This temporary boost lasted for a number of turns equal to the levels received through soul-eating. If you used it on any undead that wasn't a ghoul, though, you then lost a level per hit (dying and having your own soul eaten if you lose all your levels) permanently until you either adventured sufficiently, got zapped with Restoration or you killed enough creatures of your own race to receive twice as many levels through the soul-eating feature. On top of all that, it could talk both verbally and telepathically, detect any creature with a soul within 60 feet, grant its bearer Haste for 10 rounds 1/day, and made them 100% immune to Charm and Fear effects.
The 2e update of Blackrazor came in 2nd edition's "Return to White Plume Mountain". Its soul-eating power was pretty much identical to the original, except now ghouls weren't exceptions to the "no stabby undead!" rule, it was called out as a Neutral Evil +3 Greatsword, and it lost its special traits - sensing victims, Haste 1/day and immunity to Charm and Fear.
After that, Blackrazor and its companions languished in obscurity until near the end of 3.5; WoTC created a free update of White Plume Mountain and hosted it on their website, complete with renditions of the three White Plume Weapons as Weapons of Legacy - and a web enhancement called "Outside the Mountain" which did the same for the "lost weapon" of Frostrazor from "Return to White Plume Mountain". Alas, when WoTC reorganized their website, this free material disappeared into the ether of internet... except for the fact that DriveThruRPG salvaged it, though now charging a buck for its use. But, since /tg/ believes in stuffing that noise, you get to read it here for free, instead.
Blackrazor the Legacy Weapon is a former epically powerful soul-eating undead, something akin to an Atropal, that Keraptis summoned from another universe and bound into the shape of a greatsword. There are three Legacy Rituals that a would-be user must complete to unlock all of Blackrazor's powers; in order from Least to Greater, these are Dominating The Blade (spend 1 minute meditating with the blade and then succeed on a DC 15 Concentration check; failure costs you a negative level that is DC 15 to shake off), Opening The Soul (gain at least 2 negative levels from an undead of equal or higher CR whilst wielding Blackrazor and don't cure them except by a Fortitude save), and Exile Of The Damned (travel to the Negative Energy Plane and perform an 8-hour long ritual to attune Blackrazor to the plane).
In its default form, Blackrazor the Legacy Weapon is a +1 greatsword. Upon attuning to it, it becomes intelligent (Int 13, Wis 10, Cha 13, Ego 3, Chaotic Neutral) at level 5; it communicates through empathy, has a personality conflict whenever the bearer tries to use any weapon other than itself, and can see and hear to a range of 60 feet. At level 8, its Int and Cha rise to 16, it becomes capable of telepathic communication, and now has darkvision. Finally, at level 11, it finishes awakening and becomes Int/Ch 18, and with senses (hearing, darkvision, blindsense) to rage of 120 feet. Its Ego rises more steadily, becoming 4 at level 7, 8 at level 8, 9 at level 9, 12 at level 11, 13 at level 12, 15 at level 13, 19 at level 17, 21 at level 18, and 23 at level 20. As a weapon, it increases in power to a +2 greatsword at level 7, a +3 greatsword at level 12, a +3 vicious greatsword at level 17, and a +5 vicious greatsword at level 20. Finally, it unlocks several unique powers as it levels up.
Mental Ward: Activated at level 6, this is a +1 bonus to saves against mind-affecting effects, which increases to +3 at level 14 and +5 at level 19.
Detect Life: Gained at level 9, this allows Blackrazor to cast what is essentially a reversed Detect Undead spell three times per day.
Souldrinking: Blackrazor's most iconic power; when Blackrazor drops a victim to 0 hitpoints or below, the bearer can activate a special Death Knell as a swift action - the victim must make a Will save against the higher of Blackrazor's or the bearer's Charisma, or be swallowed into the blade. If the creature is undead, though, the bearer gains a negative level (DC 12 + Blackrazor's Cha modifier to remove) and the undead gains +5 temporary hitpoints. If the negative levels kill the bearer, then they are swallowed by Blackrazor themselves. This ability can be used 1/day at 10th level, 3/day at 13th level, and becomes automatic, activating whenever Blackrazor mortally wounds a victim, at level 17. Creatures consumed by Blackrazor cannot be resurrected by anything shy of Miracle, True Resurrection or Wish.
Haste: The last of Blackrazor's powers, upon hitting level 16, the bearer can Haste themselves for up to 10 rounds per day.
Come 4th edition, Blackrazor faded into ignominy. Although Wave appeared in The Plane Below and Whelm appeared in Open Grave, Blackrazor never received an official writeup. Instead, as part of the lead up to issue #384 for Dragon Magazine, WoTC challenged the fans to try and draft up what they thought a 4e Blackrazor would look like. The two winning entrants were featured in the D&D Alumini column for that article - of course, as that wasn't featured in the issue itself, only those able to navigate the vagueries of the internet's WayBack machine have a hope of seeing them in the flesh (so to speak). Since /tg/ believes in getting shit done, they tracked them down for you readers:
The first version was done by Zach S., who refluffed Blackrazor as a shard or emanation from Acamar the Corpse Star, a sapient black hole and one of the possible patrons of Star-pact Warlocks for that edition. A Paragon tier artifact, it was, at its basis, a +3 magic fullblade (a sort of super-greatsword first introduced in the Adventurer's Vault gear sourcebook) with the following properties: on a critical hit, it did +3d6 points of Necrotic and Cold damage, and it healed the bearer as if they'd spent a healing surge. All damage inflicted with Blackrazor is treated as being Necrotic and Cold damage. It acts as a warlock implement for warlocks on the Star pact. Once per encounter, it can sense the presence of all living (non-undead, non-construct) creatures within a 12-square radius (that's 60 feet). Finally, when used to kill a non-minion creature of Paragon tier or higher, it gives the bearer a boon based on the type of creature. An Aberration dazes the wielder until the end of their next turn, but gives them +4 Will until the end of the encounter. An Immortal grants the bearer Resist 5 to all damage until encounter's end. An Elemental temporarily stokes Blackrazor's power, meaning it now inflicts ongoing damage (5) with each strike, a state that lasts until the encounter's end. A Fae's soul gives the bearer a supercharged surge of energy, granting them Regeneration 5 until encounter's end. Natural creatures grant the bearer temporary hitpoints equal to the victim's level. Shadow creatures cause the hungry Blackrazor to sap the bearer's soul, costing them 1 healing surge or else inflicting Necrotic damage equal to their healing surge value if they have none left. And Constructs have no effect at all.
This version of Blackrazor gains +1 Concordance with its first kill of the day and -2 each time it goes a week without being fed. When Satisfied, it grants +2 to defenses and saving throws against Charms and Fear, and gains a Daily power; on a successful melee hit, the bearer can suck the victim into Blackrazor, causing them to take ongoing (15) cold and necrotic damage - if this kills them, they vanish forever. When Pleased, the defense bonuses increase to +4, it becomes a +4 weapon that does +4d6 damage on a critical hit, and it gains a second Daily power; gain +4 to Speed and +1 attack with Blackrazor each turn until the end of the encounter, at the cost of always having to move towards the closest Bloodied creature. On the other hand, if Unsatisfied, it drains 2 healing surges from the bearer at the end of each extended rest and inflicts a -2 penalty on all rolls that aren't attacks with Blackrazor. And if you Anger it... oh boy: it drains 4 healing surges from you after each extended rest, and then saps a healing surge per round that you hold it. When you lose all your healing surges, you die and become a slaughter-wight driven to carry it to battle.
Needless to say, this version of Blackrazor is a cursed weapon; unlike other artifacts in 4e, it doesn't move on, and you need to use a Remove Curse ritual to be rid of the blasted thing. Why? Well, if not for the need to constantly feed it, then because Maws of Acamar and various undead nasties will be after you constantly to take it from your cold, dead fingers.
The second version, done by Christoper T., instead makes Blackrazor a sliver of the elemental void discarded by the Primordials that became the Shadowfell. This version is a Paragon tier artifact with the base stats of being a +3 magic greatsword. It does +3d6 (+3D12 vs. Bloodied) Necrotic damage on a critical hit, grants the bearer proficiency in its use, and of course has the usual "bonus for eating souls, punishment for attacking the undead" trait. In this case, Blackrazor grants temporary hit points equal to the bloodied value of its victim when it devours the soul of a living victim. On the other hand, if you attack an undead creature with it, it drains you of health equal to your healing surge value, takes 1 healing surge (or leaves you weakened until you take a short rest, if you have none left), and heals the creature by giving it temporary hit points equal to your lost hit points. And if this kills you, then you're dead permanently. It also gives you the ability to sense all souls within a 20 square (100ft) burst at will, and lets you take a standard action as a free action 1/day.
This version of Blackrazor is far more complicated to keep happy. It loves an amoral or evil wielder (+2 Concordance), gains +1 Concordance with the first kill of each day, but gets pissed off (-1 Concordance) if the bearer uses a weapon other than Blackrazor, or goes too long without feeding it (-1 Concordance per 3 days without feeding), is seriously pissed off (-2 Cocordance) by entering the Shadowfell or being taken up by a Goodly priest, and isn't too happy if you keep it in the Shadowfell (-1 Concordance per day in the Shadowfell). When Satisfied, it grants +2 to Will defense vs. Charm & Fear, +2 to saves vs, Charm & Fear, +2 to Initiative, you can now benefit from its "hasted action" 1/encounter instead of 1/day, and can gain +2 Speed for the rest of the encounter once per day. When Pleased, it becomes a +4 weapon that does +4D6 (+4D12) bonus damage on a critical hit, it grants +4 to Will defense vs. Charm & Fear, grants immunity to Charm & Fear effects, grants +4 Initiative, lets you use a single Encounter or Rechage power from a creature whose soul you eat, can recharge the aforementioned "hasted action" by devouring souls, and its daily speed boost power grants +4 Speed. An Unsatisfied Blackrazor strips you of its hasted action power, whilst if you Anger it, it downgrades to a +2 weapon that tries to dominate you once per day (cumulative -1 penalty to the save throw each time) in order to make you kill the nearest victim - upon being fed, it calms down and restores its Concordance back to 5.
When it moves on, a Satisfied Blackrazor grants the former bearer a permanent +5 bonus to saving throws vs. Charm & Fear effects. If Unsatisfied, however, it vanishes in the middle of the next fight, stealing bloodied value hitpoints and a healing surge in the bargain.
In 5th edition, meanwhile, Blackrazor moved up in the world by being part of the Dungeon Master's Guide alongside Wave and Whelm - poor forgotten Frostrazor was left out again. The 5e version of Blackrazor is a +3 intelligent magic greatsword which will only allow non-lawful characters to attune to it. In 5e, its iconic "Devour Soul" power is treated this way: when you kill a non-construct with Blackrazor, it eats the creature's soul - not only does this prevent the creature from being revived by anything short of a Wish, it grants you temporary hit points equal to the victim's maximum HP for 24 hours, during which time you also have Advantage on attack rolls, saving throws and skill checks whilst you have Blackrazor in hand. Killing multiple victims doesn't stack the bonus, it just resets the clock. However, if Blackrazor strikes an undead creature, you take 1d10 Necrotic damage and the creature is healed for 1d10 hit points; if you drop to 0, your soul is eaten. It also has the Soul Hunter property (you can sense the presence of any Tiny or larger non-construct, non-undead creature within 60 feet, you are immune to Charm and Fear) and can cast a Haste spell on you 1/day - if it does, Blackrazor sustains the spell for you, so you don't need to make a concentration check. This Blackrazor is Chaotic Neutral with Int 17, Wis 10, Cha 19, and can hear and see (with darkvision) to a range of 120 feet. Blackrazor only cares about getting fed, so it provokes a personality conflict if you go three days without feeding it, making its complaints felt at the sunset of the third day without a meal and each day thereafter until fed. Interestingly, it is noted in the DMG that it seems to have developed a fondness for Wave and Whelm, and seeks to reunite with its other 2 counterparts. (Presumably because normal magic weapons aren't very good conversationalists.)