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). It, and 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.
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.
Glaive
Released in Book Two: Massacre, the Glaive has a big Volkite Carronade, which actually causes hits to anything that falls under its inch-wide beam (which would be six feet if scaled up).
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 turbolaser-looking (to be clarified when the actual book is released) gun mounted in a rigid casemate.