Mutilator
The Mutilators are massive horrible abominations of flesh and metal, who live only to RIP AND TEAR. They are often called melee-oriented Obliterators, but though crunch-wise (and model-wise, to the extent of having the exact same torsos and legs) they are practically identical, fluff-wise they are nowhere near.
Fluff
Mutilators were once Terminator champions, who were so obsessed with close combat, that they merged with their weapon into one being. Apparently, spending 90% of your time in the fucked up hell-dimension of the Eye of Terror you can do such things without any daemonic techno-virus help. So now their bodies are covered in grown-in terminator armour, power weapons, chain weapons, lightning claws, and various other sharp objects, as well as their souls, which have merged in all of the machine-spirits of all the weapons they have absorbed. Unlike Obliterators they still can speak non-binary language, but being absolutely batshit-insane butchers they rarely have anything to tell.
Crunch
The guy that wrote this codex seems to have a serious problem with internally balancing his works. For example, his most celebrated work, Codex Dark Eldar in 5th edition had a handful of units that were terminally useless versus much more useful choices. Sadly, this is exactly what Mutilators are - useless point- and slot-sinks that cannot compete in their segment. Let's take a closer look at them, shall we?
PROS
- 2 wounds and attacks
- 2+ armour save from Fleshmetal
- Daemon (5++ and causes fear)
- Deep Strike
- Come with the following weapon pairs: Power sword/axe/mace, Chainfist, Lightning Claw
- Fairly inexpensive at 55 pts./model
CONS
- They have Slow and Purposeful for no good reason at all, because...
- No ranged weapons, which means...
- Deep-striking these guys will see them offed by a volley of plasma/melta shots, which wouldn't have been a big deal, except...
- Unit size is capped at 3 models max!
- Additionally, they have to change weapons every turn. Basically, if they fail to kill something in a round of combat, depending on their target they will likely be less effective against them next round.
- Much like Warp Talons, they also lack assault grenades, meaning they will have lots of trouble tackling cover-camping foes (although they can handle it better due to their tougher armour).
- They take up an Elites slot, which has both cult marines and more cost effective options namely Possessed and the far more preferential Chaos Terminators. ALL of these can do what Mutilators do and can either do more or do it better or both.
Conclusion
They could have been sweet! Really, they could have been a nice go-to assault unit if they were less Obliterator and a little more Possessed/Terminator. If instead of Slow and Purposeful they had fleet or outflank or something that could otherwise get them into close combat faster, that would have helped tremendously. Failing that, if they had a higher unit cap could withstand punishment better or perhaps assault weapons (leaving obliterators to take even more of a heavy weapon ranged role), that would have helped too. However, we're left with a unit that has been effectively hamstrung such that it doesn't step on other units' toes in the crowded elites slot.
On the other hand, a a single Mutilator with the Mark of Nurgle is a cheap, easy-to-deep-strike, annoyingly-tough-to-kill distraction. Does your enemy waste plasma and las fire, railguns, Vindicator rounds et al removing a single two-wound model, or does it let it get away with occupying squishy backfield units in melee and/or ripping and tearing fortifications and Basilisks new anuses? Does your enemy disrupt it's carefully layed-out battle lines and possibly waste turns of fire for units with heavy, salvo and ordnance weapons to avoid this one guy? Is this cheaper, more disposable and just as effective as 3 Terminators? Will your opponents allow you to use an Oblit as a counts-as? Why not find out for yourself?
They are also suitable to bastion-breaking, being able to lay in with multiple chain fist attacks and can be thought of as a kind of "Chaos Assault Centurion" in that regard. They are also useful as bodyguards for HQ units that aren't directly supporting another, lesser unit.