The Mutilators are massive horrible abominations of flesh and metal, who live only to RIP AND TEAR. They are often called melee-oriented Obliterators, since crunch-wise they are practically identical and model-wise they are built from the same kit. Fluff-wise, however, they are nowhere near.
Mutilators were once Terminator champions, who were so obsessed with close combat, that they merged with their weapons and armour 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.
The guy that wrote this codex has some interesting ideas when it comes to internal codex balance. For example, his most celebrated work, Codex Dark Eldar in 5th edition had a handful of units that were generally to almost entirely useless versus transparently better choices. Mutilators certainly give off an aura of the former - let's take a closer look at them:
- 2 wounds and attacks (Stocky and an alright number of strike per turn)
- 2+ armour save from Fleshmetal
- Daemon (5++ and causes fear, but vulnerable to Grey Knight shibozzle)
- Deep Strike
- Come with the following weapon pairs: Power sword/axe/mace, Chainfist, Lightning Claw, which cannot be used twice in a row (the Chainfist is straight-up better than the axes and the Claws for the swords, so they're just back-ups for protacted combats)
- Fairly inexpensive at 55 pts./model
- Access to the full gamut of Marks of Chaos, at a slightly more expensive cost than the norm. While Slaanesh is useless, Khorne and Tzeentch are alright and Nurgle is *amazing* as it prevents Krak missiles and lascannons from IDing them
- They have Slow and Purposeful, which robs them of running and sweeping advance to grant them Relentless, which they don't need because...
- They have no ranged weapons - so ...
- Deep-striking these guys will will open them up to a volley of fire without any sort of counter-attack, compounded by the fact that . . .
- Unit size is capped at 3 models max!
- Additionally, they have to change weapons every turn. Thanks to the aforementioned weapon redundancy this isn't too much of an issue unless the best weapon against a target, in a protracted combat, is power mauls (low armour save, highish Toughness, Initiative vital? when will this happen) in which case Mutilators will be screwed on the off-turn
- Much like Warp Talons (and a plethora of other units, certain Tyranids), they also lack assault grenades, meaning they may have trouble tackling cover-camping foes that they aren't attempting to Fist (although they can handle it better than most examples due to their tougher armour). Bear in mind of course that Mutilators generally go after high armour save targets, who are less likely to bind themselves to cover than most.
- They take up an Elites slot, which has both cult marines (if the army isn't led by a correspondingly marked Lord, which is a given for armies planning to use one sort - basically this is only an issue for armies with multiple cult units) and options such as Possessed (that bastion of reliable design), Chosen (the poor man's Sternguard) and the versatile but often suicidal Chaos Terminators.
On the whole, a unit that is very limited in its combat usage but for its cost pretty dang tough, and outright solid with the Mark of Nurgle. If used as a typical combat threat, the unit will generally perform poorly - it lacks speed, it has no shooting for pre-combat and it doesn't even have grenades. Instead, by making the most of its Deep-Striking capacity and combined great tough-to-cost ratio and combat flexibility, a a single Mutilator with the Mark of Nurgle can act as 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 he ignore it and let it get away with grarbling squishy backfield units in melee and/or ripping and tearing fortifications and Basilisks new anuses? Does your enemy disrupt its carefully layed-out battle lines, possibly wasting turns of fire for units with heavy, salvo and ordnance weapons to avoid this one guy? And as for the potential user, one must ask: is this cheaper, more disposable and just as effective as a trio of Terminators? Will your opponents allow you to use an Oblit with the guns magnetized off as a counts-as? Why not find out for yourself?
Oblits and Muties are still complimentary this edition, in that they both lose their classic "choose a weapon each turn" rule for one single weapon with randomized stats. In the Mutilator's case, it's WS3+, with Fleshmetal weapons of S:User+D3, AP:-(D3), D:D3, roll separately for each (their base strength being 5). Whereas a lot of people are unhappy about Oblits being stuck with close-range weapons that are at best slightly worse than a Lascannon, mutilators are probably just happy about the removal of Initiative and Initiative Steps so they can strike first with what amounts to a power fist. They're still slow with a puny Move of 4", but "having trouble getting into assault" is something they're used to now. Also they're rocking 3 wounds apiece now.