Fellblade
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.
Variants
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.
Fellblade
The "original" Space Marine super-heavy, released in Betrayal. Its primary weapon is the 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. Has double twin-linked lascannon sponsons as well, for extra dakka.
Glaive
Released in Book Two: Massacre, the Glaive has a 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 quite nicely by 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.
Falchion
Sure enough, announced as part of the run-up to Book Three: Extermination, we have the Falchion Super-Heavy Tank Destroyer. 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 a gun that huge are too bulky to mount in a turret.
Warhammer Fantasy
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 (inscrolled 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 a reason: 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.