Lord of Skulls
"MAKE PEE-NIS INTO ROBOT!"
The Lord of Skulls (formerly known as the Lord of Battle, colloquially known as The Rape Train) is a massive Daemon Engine of Khorne. Those tanks on its back store the blood of murderers, which get heated to boiling by the daemon's rage and then vented out through the chest-cannon (with various ranges, strengths, and so on depending on which one you get). Its right arm holds the Great Cleaver of Khorne, a destroyer melee weapon. The left arm is either a gatling cannon or (as pictured here) a skullhurler, a cannon shaped like a skull that shoots a bunch of skulls that gnaw on their targets (forcing successful saves to be re-rolled). Seriously, we couldn't make this up if we tried -- just imagine skulls chomping everyone like Pac-Man.
He started out as an Epic model; when Games Workshop released the new Apocalypse book for 6th Edition, they turned it into a 28mm-scale model (although the new one is more a hybrid with the Death Dealer, since it inherited its humanoid front section and chest-mounted cannon). The modeler actually lost count of how many little skulls he put on the thing. The resulting model looks really, REALLY, stupid, and/or harkens back to Warhammer 40,000: Rogue Trader. (Same thing, really.)
Forge World sells a conversion kit for turning this model into a walker. Either you think it makes it look generic as fuck, or gives some kinda 'realism' and Khornateness to the model without going over-the-top.