The Fellblade Super-Heavy Tank is a Space Marine tank based on the Baneblade chassis, used during the Horus Heresy. Like the Predator Tanks of that era, it has a bubble-top turret (like the T-55 and any later Soviet tanks), though it also built with a more durable internal structure and power plant (apparently, it took nearly thirty thousand years to get the secrets of arc reactor technology from Stark Industries). This tank, along with many other tank classes during the Great Crusade, was equipped with a Flare Shield. This is basically the E-Web from Babylon 5; it reduces the energy (kinetic or otherwise) from concentrated strikes and it spreads any damage out across the armor and shields. It's a lot more potent than it sounds (seriously, think about it - the reason the bullets and weapons generally work at all is by focusing energy onto very small points. It takes a lot of energy to punch through solid metal, but you generally only need to bust through a small area of it to deliver murder to the delicious squishy internals. The smaller the area you can deliver force to the lower energy needed to reach punch through because that's how pressure works, force over area. So if the shielding can increase the effective surface area of projectiles even by small amounts it hugely decreases the rounds ability to punch through armor.)
On the other hand, you might have been here looking for the other Warhammer item of note by this name, the legendary warpstone-gromril blade of the Skaven that was used to fell Nagash. See the bottom of the page instead.
Model and Rules
Games Workshop first released rules for Space Marine super-heavy tanks in a White Dwarf expansion to Space Marine (the game that later became Epic); these were the Glaive, the equivalent of the Baneblade, and the Falchion, a Shadowsword equivalent.
The Fellblade name first appeared in the Horus Heresy collectible card game, around the year 2005, though it would not receive rules until after the release of Apocalypse in 2007; inspired by the inclusion of Baneblades and other super-heavy vehicles in 28mm-scale Warhammer 40,000 games, Bell of Lost Souls wrote a datasheet for the Fellblade, based on the Baneblade datasheet. It was generally well-received.
Around 2010, some guy called Machinator wrote updated rules for four Fellblade variants - the Glaive, the Lance (anti-tank las/plas variant), the Broadsword (anti-infantry/fast dakka/flamer variant), and the Warmaul (anti-fortification giant Demolisher Cannon variant). He did this partially because it's cool, and partially to sell bits from his eBay store.
In 2012, Forge World released a line of books and models tying into the Horus Heresy era, and the Fellblade is part of their first wave of models, coinciding with the release of the first book, Betrayal. It looks like a Baneblade with smooth sides and a larger engine (presumably representing the ceramite armor and arc reactor, respectively), topped with a monster Deimos Predator Tank turret with a twin-linked rape cannon on the top. And that's in addition to the hull Demolisher Cannon, the twin-linked hull heavy bolters, and the two quad lascannon batteries (just like ones on the Spartan Assault Tank), for a total of THIRTEEN BARRELS OF HELL (all for only 25 points more than a vanilla Baneblade, if you don't spring for the +1 BS upgrade). If that's still not enough guns, it can also be outfitted with hull-mounted Combi-weapons, a Havoc Missile Launcher, and a Hunter-Killer Missile Launcher. Marine fanboys everywhere wet their pants.
Like the Baneblade, there are a few Fellblade variants; unlike the Baneblade, Space Marines are creative with naming their super-heavy tanks, as opposed to rolling dice which say "Bane," "Sword," "Storm," "Shadow," "Blade" and "Hammer" on them.
The "original" Space Marine super-heavy, released in Betrayal. The Fellblade was most notable for its use of both Mechanicum atomantic arc-reactor technology and a reinforced metaplas alloy chassis superior to that of the Baneblade. It mounted a twin-linked Accelerator Cannon as its primary weapon and would also take to the field equipped with a suite of secondary weapons: a demolisher cannon, sponson-mounted quad-lascannons or laser destroyers, hull-mounted twin-linked heavy flamers or heavy bolters, and a variety of pintle-mounted weapons.
At 302 tons, the Fellblade is lighter than the Baneblade. This can be attributed to its reinforced metaplas alloy which seems to be far lighter than, but just as strong as (pound for pound), the ceramite armor found in most Imperial vehicles. This is akin to advanced composite armor in comparison to conventional homogeneous rolled steel.
Rules-wise, its primary weapon is the above-mentioned twin-linked turret-mounted Accelerator Cannon that can either fire high-explosive shells comparable to a super-charged Battle Cannon (S8 AP3 7" Blast) or armor-piercing shells comparable to the Vanquisher battle cannon (S9 AP2 3" Blast, rolling 2d6 for armor penetration), so it's good for nearly any situation. Also comes equipped with a demolisher cannon and twin-linked heavy bolter. It has double twin-linked lascannon sponsons as well, for extra dakka, which can be switched out for laser destroyers if you have to fight another superheavy. Fancy enough that Argel Tal, arguably the Word Bearers' most senior Chapter Master, had one as his personal tank.
Released in Book Two: Massacre, the Glaive Super-heavy Special Weapons Tank or Fellglaive, was a variant of the Fellblade, armed with a volkite carronade, designed to destroy xenos beasts and enemy light vehicles in a single overwhelming shot. Its primary weapon is so ridiculously overpowered that the Techpriests of Mars only agreed with some acrimony to the Emperors demand that only a tank could be armed with it. 'Cause even the Imperium have standards.
Though all eighteen Legiones Astartes received limited quantities of Glaives, the Salamanders and Dark Angels were noted to operate them more regularly. Its rarity was because the Glaive was an overcomplicated trainwreck to produce and cost an equally ridiculous amount of Imperial moolah. The Dark Angels were given specialized Fellglaives for use in the Dreadwing which were designed by the Emperor's greatest armorers. These vehicles were equipped with Vortex Weapons in place of its volkite carronade, because they're Dark Angels, why the fuck not?
On tabletop, the Glaive has that big Volkite Carronade, which fires a big beam that hits everything in front of it at S8 AP2; combined with Haywire and the Deflagrate ability inherent to Volkite weaponry, its short range is compensated for quite nicely with its knack for vaporizing anything in front of it (unfortunately, this includes any allied units which happen to be in the beam's path, so use carefully). In addition, should it hit a Super-Heavy or Gargantuan Creature, they take an extra D3 hits. Doesn't have a Demolisher, though.
Sure enough, announced as part of the run-up to Book Three: Extermination, we have the Falchion Super-Heavy Tank Destroyer. Also known as the Mammoth in a clear and blatant nod to a particular RTS game.
The Falchion uses a potent weapon which combines technologies fielded in the Fellblade and the Shadowsword super-heavy tank. It's got a twin-linked Volcano Cannon guaranteed to ruin the day of any Titan or super-heavy tank that gets in range. Like the Glaive, it misses out on a Demolisher and twin-linked heavy bolter, though unlike the Glaive, it must bodily aim its Volcano Cannons at the target: the capacitors for the twin cannons are too bulky to put inside a turret. The miniaturization of the Falchion's Volcano Cannon's power source as well as the fact that they need to twin-link it, made the Falchion a stupidly expensive investment even when compared to its sister tanks, so don't expect to see that much of them in the 41st Millennium.
Interestingly enough, the Falchion was developed long before the outbreak of the Horus Heresy and as such its prey was not the enemy Titans it would later see such extensive combat against. As the Great Crusade expanded ever outward, the Expeditionary Fleets encountered a staggering array of foes, some of whom were of a truly gargantuan scale, but all were turned to molten swiss cheese once this tank rolls onto the fields.
In Warhammer Fantasy Battles, the Fellblade is the name of a legendary magical artifact, a sword crafted by the Skaven Grey Seers from a cocktail of warpstone and gromril, bathed in the most potent killing curses the Grey Seers could invent (inscribed with runes so deadly simply reading them would kill the reader). The result, in the game, is a weapon regarded as one of the most outright killy in existence, so deadly even holding the thing will eventually kill whoever is wielding it.
The Fellblade was created for one purpose, and one purpose only: to kill Nagash, who at that time was preparing to perform his Great Ritual, which eventually reduced Nehekhara into the land of the dead that it is today. Also Nagash was sitting on a big vein of Warpstone, which was the reason skaven wanted the Great Necromancer dead in first place. Using a human pawn, the Skaven succeeded, inflicting the first death of Nagash. In fact, they did even better than they knew; according to the 8th edition, the Fellblade not only killed Nagash, it kept on killing him, leaving a curse that meant each subsequent reincarnation of Nagash was weaker and weaker. For this reason, the Fellblade was one of the artifacts sought to resurrect Nagash before the beginning of The End Times, with Mannfred von Carstein, Mortarch of Shadow, eventually recovering it during The Battle of Mordkin Lair. During the dread ritual that restored unlife to the Great Necromancer, the Fellblade was destroyed, undoing its baleful effect on Nagash's spirit.
The most recent rules for the Fellblade were in the 7th edition Skaven armybook. It's a magic weapon that costs an insane 100 points with the following effects: attacks from it are Strength 10 and force a reroll of successful Ward saves, with each unsaved Wound being multiplied into D6 wounds, but the wielder must roll a D3 on the end of each of his turns; on a 1, he suffers 1 Wound with no armor saves allowed.