- The Nightbringer reminding how terrifying he can be
"When the Grim Reaper comes to call, words fail- they're just too small."
- – Dixie Lyle, To Die Fur
"Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it."
- – Terry Pratchett, Reaper Man
Aza'Gorod, AKA The Nightbringer, Warhammer's Grim Reaper, the Necrons' Mortarion, Has No Friends or FUCKING CHEESE! is one of the main C'tan gods in Warhammer 40k alongside the Deceiver and the Void Dragon that is still relevant enough to be talked about. The other C'tan gods have been eaten, shelved in a GeeDubs corner, or are just hiding in a cupboard somewhere.
Spooky Story Time
The Nightbringer is the C'tan most directly responsible for unleashing the Necrons on the galaxy. His feeding on the star in the Necrontyr home system caused the solar radiation output that gave the Necrontyr such short, cancer-ridden lives. This in turn caused their obsession with death and spurred them to colonize other worlds. These interstellar voyages led to the necrontyr encountering the Old Ones, and the disparity in health and lifespans caused the necrontyr to resent the Old Ones and wage war against them. All this led to contact between the C'tan and the necrontyr, and in some accounts the Nightbringer was also the first C'tan discovered. Finally, their hatred of the Old Ones and wanting to escape their sickly biology are the two main reasons the necrontyr leaders accepted the C'tan's offer to be roboticized into the Necrons.
During the War in Heaven, it was the entity that single handedly enforced the concept of the fear of death or at least the Grim Reaper in the 40k Galaxy. He, along with the Outsider, another C'tan, was infamous for being a team killing fucktard during the War in Heaven after he and the Outsider were tricked by both the Deceiver and Cegorach to eat their fellow C'tan brethren in order to gain more power. As you can imagine, things did not go too well for the Necrons and the C'tan during that incident, making the Deceiver one of the biggest walking fuck ups in 40k history.
Consequently, the Nightbringer, alongside the rest of the C'tan (save for maybe the Void Dragon and the Outsider), were stabbed in the back by their former slaves, the Necrons, subsequently being turned into fucking Pokemon after getting blasted into a million pieces. You can say all you want, but at least Khaine was treated with dignity and respect by the Eldar after he was shattered by a bajillion pieces unlike the poor C'tan who have turned from one of Warhammer's most sinister forces into a fucking joke.
To give an idea of how low the Nightbringer has fallen, he got basically stared down by Uriel Ventris who told him to piss off while threatening to blow the both of them up, with the Nightbringer realizing Uriel wasn't afraid to die just to make him suffer. And that was prior to the sharding retcon; it's only gotten downhill from there.
On the Tabletop
Before the Warddex, the Nightbringer was the hardest thing to kill outside of Apocalypse with toughness 8, five wounds, and a 4+ invuln save (Remember when that was impressive?). As a monstrous creature with S10 and loads of attacks that ignored invul saves, getting near him was a death sentence. The Warddex removed it entirely in favour of more customisable C'tan Shards. It was re-introduced in 7th edition, in a weaker state, in an edition with rampant power creep and in a codex with the strongest Necron roster yet. The C'tan went from being one of the most cheesy units Necrons had, to one of the weakest. Noticably a Relic in the Shield of Baal makes the Nightbringer Toughness 8 instead of 7, making it no longer die to boltguns, which is sort of a big deal if your opponent isn't playing Dark Eldar.
8th Edition is the return of the Nightbringer. While still at toughness 7 with 4+ save and 8 wounds, he is now a character and thus unable to be shot at if he is not the closest unit. This adds so much to his survivability and he can now actually make it to close combat. However, the biggest change is that now the Nightbringer can Rip and Tear virtually anything. His weapons are Gaze of Death (Assault D6, 12" range, -4 AP D3 damage) and Scythe of the Nightbringer (AP -4, D6 damage). Both of these weapons hit on 2+, thanks to the Nightbringer's Ballistic/Weapon Skill, and wound on 2+ thanks to their own special rules. On top of that, he gains 1 Power of the C'tan (now 2 powers with the Codex). The most useful one being Antimatter Meteor, which is basically a 24" range Smite. Time's Arrow can also be useful in certain situations where you want to snipe out an annoying character. Seismic Assault seems the weakest of the three powers, so it is not recommended to ever take it.
Antimatter Meteor requires a 2+ to cast, and deal D3 mortal wounds to the closest unit, or D6 if the casting roll was a 6.
In short, a 1/6 chance to fail to cast (0 damage), a 4/6 chance to deal D3 (average 2 damage), and a 1/6 chance to deal D6 (average 3.5 damage).
Average damage per use = 1.92 mortal wounds
Seismic Assault deals a mortal wound on a roll of 6 and rolls one die per model in the closest unit.
In short, a 1/6 chance to cause a mortal wound per model in the unit.
Average damage per use against a single model = 0.17 mortal wounds
Average damage per use against a unit containing 11 models = 1.83 mortal wounds
Average damage per use against a unit containing 12 models = 2 mortal wounds
Antimatter Meteor is expected to perform better against units with less than 12 models, meaning that (in most cases) it should be preferred ahead of Seismic Assault.
Note: With its lower dice count, Antimatter Meteor additionally has the potential to benefit more than Seismic Assault from a stratagem re-roll.
Bottom line, if you play Necrons you always take the Nightbringer. No questions asked, he is too good not to take and he even costs just 230 points (or 12 power, if you play Power Levels), which is nothing for what he offers in offensive power.
DoW Dark Crusade
The Nightbringer also appeared in Dawn Of War: Dark Crusade, sorta. The Necron Lord has the ability to turn into a manifestation of the Nightbringer, which skyrocketed his DPS (not as much as what Relic units had, but still high - enough so that when executed properly and against careless players to don't move their units out of range of his scythe, the Nightbringer will flat out kill any of the other race's Demon/Relic units before his timer runs out), but more importantly, he was INVINCIBLE while the ability was active, meaning the best any poor sap facing it could do was tarpit it to hell and back until it wore off and hope it didn't kill anything too valuable.