Obliterators
These big motherfuckers are Chaos Space Marines who have been infected with the so-called "Obliterator Virus" (Oh so subtle GW...Oh so subtle), which basically turns them into a giant ass walking Daemonic Engine of Mass Rape. It's a large walking blob of Termie armour, daemonically enhanced killing machine, and guns ...BIG guns.
Obliterators are rare (usually) and very sought after, due to their complete ability to rape everyone and every thing within a 5 mile radius. They don't work for any particular legion or worship any Chaos God individually, but they have their own insider cult with the pursuit of knowledge, alchemy and the fusion of flesh and Stehl. Usually having a connection to techmarines or techpriests they get some KICKASS powers along with the virus, however it also turns them batshit insane. Any object they hold can get merged into them, allowing them to copy them AND produce ammunition for them....Tyranid Digestion Pool?! (Which is kind of fucking creepy when you didn't know you were infected) The metal they get infused with turns them durable as fuck, (Wolverine anyone?) although very slow, but they have teleporters so it's all good. The Iron Warriors have a shit ton of these guys just waiting for commands due to some connection with the Obliterator cult.
The Mutilators are the melee-oriented cousins of the Obliterators. Instead of being covered in guns, they're covered in power weapons, chain weapons, lightning claws, and various other sharp objects.
Overview
Obliterators are probably the most used heavy support choice in the Chaos Space Marines 4th and 5th edition Codex, and with good reason. During each shooting phase, they have a choice of using a twin-linked Flamer, a twin-linked Plasma Gun, a Plasma Cannon,a twin-linked Meltagun, a Multi-Melta, or a Lascannon, meaning it matters not what the enemy is, they have a means of dealing with it. They also have Powerfists and a 2+ armor save and 2 wounds, though since they can only be fielded in groups of three at the most, it's best to keep them out of melee, since you want them to free to shoot at enemies.
Their drawbacks are that aside from an extra wound, their statline is the same as a regular Chaos Marine's, meaning they can be hit with Instant Death from a number of heavy weapons. Like Terminators Thousand Sons, they also have Slow and Purposeful, so it can be a problem getting them where you need them. Also, all Obliterators in a unit have to fire the same weapon, no mixing and matching, so make sure you know what weapon you want to use.
Oblits were outrageously more dangerous in the 3rd edition Codex due to their S/T5! However, their toughness got FAQ'd down to 4(5) and although they didn't have a plasma cannon or multi-melta, they had access to heavy bolters and frag missiles. Obliterators gained infamy through the notoriously broken Iron Warriors list that spammed them and used Basilisks to decimate anything thrown at them before it could even shoot back. It's no exaggeration that these guys were among the reasons that the 3rd edition codex was considered to be so overpowered that GW had to release 3.5 codex to balance it.