"Tank: So what do you need? Besides a miracle."
"Neo: Guns. Lots of guns."
- – The Matrix
These big motherfuckers are Chaos Space Marines who have been infected with the so-called "Obliterator Virus," 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. The first Obliterator was created when the Iron Warrior Volk was possessed by Sa'a'ram, a daemon of Khorne that claimed to have been born when the first mortal built a weapon with the intention to kill. The virus itself was rare case of brotherly cooperation between Perturabo and Mortarion. Speaking of Perturabo, it's all but stated in 40k fluff that his daemon prince form is basically an obliterator but bigger and with more guns.
Obliterators are rare (usually) and very sought after, due to their complete ability to butcher everyone and every thing within a 5 mile radius. The process of becoming an obliterator is as with everything 40k, horrifically Grimdark. At first you don't feel much different. Your accuracy starts improving and your weapons of choice feel better in your hands, so much so you ask yourself, why would I ever put them down? Soon after you are given the answer - YOU CAN'T. Things start to get a bit freaky as you absorb your first weapon and begin to produce ammo for it. Hope you were comfortable in your armour too, as now it's part of you! Flesh melds with plasteel and joints remake themselves where the virus pleases, any sanity the chaos marine may have still had is lost as they find their mouth is now a meltagun barrel. 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, as they willingly undergo the transformation, for the legion has mastered and improved upon the virus, making it a much more controlled transformation where the end result is slightly less of an insane fleshy mess. You can now spew that Dakka as a true Iron Warrior would. Many of these obliterators were once the legions techpriests and warsmiths, who spend most of their time building dinobots and other such mechanical-daemon monstrosities Though judging from BL, some Grand Companies would rather lock their Obliterators in a vault rather than leave them free to do crazy shit.
For years they had the dubious (and shared, with Mutilators) honour of being the ugliest models in the Chaos line, though that's to be expected when you've got a ton of clunky machinery directly underneath your skin. It wasn't until 2019 where they finally hit the gym and got SWOLE, with the brand new minis towering over their previous incarnation, and with far more Dakka spewing out of them!
Oblits were first introduced to 40K back in the early days of third edition. The basics were already in place at that point, being the large profusion of heavy and special weapons otherwise on a terminator's profile, including saves and weapon portability. When first introduced they could not charge or otherwise engage in an assault but otherwise had power fists along with their projectile weapons for some security in close combat.
Oblits were outrageously more dangerous in the 3.5 edition Codex due to their S/T5! However, their toughness got FAQ'd down to 4(5) (meaning at the time no immunity to instant death S8) 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 4th ed codex to balance it.
4th and 5th
Shuffling back into an HS slot, Obliterators were 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 Heavy 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 (plus a 5+ Invul) 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 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.
6th and 7th
The 6th edition book, while trying adjust units to avoid the problems where the 4th edition had some units that were so good it was pointless to use anything else, actually gave some improvements to the Obliterators. They can now be given Marks of Chaos, and they can finally use Assault Cannons (though rending took a hit). The only drawback they gained was that they can't use the same weapon two turns in a row, which isn't all that big a deal, especially since they have some overlapping weapons (Use a plasma cannon one turn, and then TL plasma guns to clean up the next). Though improvements to other units does mean that now they do actually have some competition in the shooting area.
This edition has had a number of changes to the unit over time. Most notably, the ability to select basically any gun ever is replaced with a generic 'Fleshmetal Gun' that has randomized Strength, AP and Damage. A S between 7 and 9 is fine, 24 inch range is fine, but their damage is a randomized D3, but Multi-meltas and Lascannons -- the weapons they really want to use -- are randomized damage D6 with better AP. The release of the Chaos Space Marines codex buffed them slightly by changing the Fleshmetal Weapons from Assault 2 to Assault 4, doubling their firepower at no additional points cost. Also, their weapon damage is generated before they shoot along with the rest of their random stats, meaning that if you get Damage 3, the entire unit will have Damage 3 shots. That's 36 potential wounds right there.
Even bigger, the CSM Daemonkin book transformed them once more granting them stat bonuses (like T5 and 4 wounds), boosting Fleshmetal guns to Assault 6 and giving them an S+1 AP-1 DD3 close combat weapon. Additionally, they can get buffs from a Master of Possession or Greater Possessed if you're in a pinch. And best of all, they got these kick-ass new models!