Heldrake
Heldrakes are a type of Daemon Engine utilized by the Chaos Space Marines in Warhammer 40,000. Introduced in Sixth Edition, they were immediately the center of a deliciously skubtastic debate.
Heldrakes are giant, metallic, fire-breathing, Chaos dragons/jetfighters. They used to be regular Space Marine aircraft, but they've been warped by the power of the Immaterium. The pilot(s) remained trapped inside, only to have his body consumed and his soul bound to the daemonic machine; in fact, the Heldrake's roar is actually the amplified screams of the former crew. They travel trough the Warp clinging to their warband's ship, before soaring into battle like a bat out of hell (quite literally), and wreaking unholy havoc (Word Bearers approvedTM).
This is, of course, really awesome fluff, but it's from Phil Kelly, so that's not exactly a surprise.
The model itself has a FATAL-size anal hole. Many fa/tg/uys find this absolutely silly, although it is based on an old Epic 40,000 model, so...
Crunch-wise, Heldrakes are hilariously overpowered the basic flying unit, in a book that is generally pretty weak. In fact, Heldrakes are considered pretty much the only thing keeping Chaos Marines competitive nowadays. Fielding more than one (sometimes even only one) Heldrake in a game is the fastest way to have your opponent bitch about the only thing in your Codex that can keep you on the board and have him ragequit. If you field three or more you automatically are That Guy.
And Then, 7th Edition
With the arrival of 7th Edition and the corresponding FAQ, the Heldrake took several massive hits in its brazen demon balls that might throw it down from the mighty spire of cheese it so proudly dominated before. First, the rulebook changed the rules of the Vector Strike USR to only do one AP 2 S user hit on normal troops, while enemy fliers take D3 instead, and after that, the recent FAQ nerfed the Heldrake by removing the turret rule to the flamer weapon, so it now have to shoot from the tip of the weapon, aka the head of the drake. While this might not make it unusable, it does nerf it to more logical levels for a flyer.