Decimator
Decimators are an unaligned Daemon Engine and look like badass versions of Daemon Knights (AKA FUCKING AWESOME!). Decimators are armoured with slabs of tainted ceramite and adamantium, and a low-set ‘head’ which gives the Decimator a characteristic hunched posture which exudes brooding and vicious menace.
Overview
Decimators are a hellish amalgamation of human and xenos technology fused and brought to unholy life by the darkest Warp sorcery and dickery. While they are rarely encountered due to the amount of fusing between compatible human, xeno and daemon technology, when they appear, you better clench your butts. If you want to find them, go to the Galactic South of the Segmentum Tempestus in the benighted reaches of the Nightmare Rifts. Here in the dark void beyond the borders of the Imperium, some unknown source, it is believed, barters these dread killing machines for a high price in blood, plunder and souls - meaning that they are a not only a Daemon Engine, but a mercenary Daemon Engine who will pledge loyalty for the biggest prize of gore, loot, and ballbusting sex. Fucking Awesome.
Crunch
Decimators, being the badass motherfuckers they are, are able to carry and field a wide range of terrible and rapetastic armaments, such as rapid-firing Butcher Cannons, multi-barrelled Stormlasers, Soulburner Petards, Heavy Conversion Beamer, and great raking Siege Claws, to name but a few. They have been something of a tanky unit since their inception, sporting rules and stats granting additional resilience.
And did we mention that they look fucking cool?
Origins
There's discussion that the Decimator originated in the dark days of 2nd ed. as a sort of wheeled contraption, possibly in Epic 40K. They functioned somewhat like a Blood Slaughterer in crunch and size, if any data can be found about them.
5th Edition
This one that we're talking about here had a very late 5th-edition debut. Towering over even the mighty Contemptor Pattern Dreadnoughts of the Adeptus Astartes, the Decimator is believed to be an attempt to bolster the ranks of such ancient devices that still serve the Traitor Legions. If this is the truth, then it is feared that many warriors of the Imperium will fall before these deadly war machines.
At least, that's what GW would have you believe back in the day. Crunch-wise, they debuted with crappier Dreadnought stats (12/11/11 armour, WS/BS/I 3) and all the usual daemon engine goodies like ignoring shaken and stunned results. They could also regenerate blown weapons and unimmobilize themselves or could stand back up in a very Necron-esque way - all part of a special rule called Unholy Vigour. For weapons, they came with siege claws, which were basically dreadnought lightning claws that could falcon punch buildings and vehicles with 2d6 to pen and then could deliver heavy flamer hits on any transport occupants. These could be replaced with a butcher cannon, which at the time was a 4 shot S8 Reaper Autocannon; a Storm Laser for melting MEQs; a Heavy Conversion Beamer for some reason or a Soul Burner Petard, which dropped rending pie-plates. Furthermore, they could be marked by the gods in an era where that had long been swept into the bin - a very exciting thing, although it required an independent character in the same army with the same allegiance. Khorne gave 2 attacks on the charge, Nurgle gave rerolls to unholy vigour, Slaanesh gave defensive grenades and Tzeentch rerolled shooting to-hits of 1. Obviously, Nurgle and Tzeenetch were the mainstays.
So... a tad underwhelming at 155 points and its touted resilience wasn't that hot. There was one golden daemon diorama that featured a Decimator fighting a Blood Angels Contemptor and given the stats at the time, the Contemptor would have blown the Decimator before it even got a swing. It was a cool niche model and it actually had some utility as a gun platform at the time and it's mainly what people used them for as an Elites choice in a Daemons or CSM army. In the Daemons at least it was a pretty neat dakka piece, outright better than the Soul Grinder but was then overshadowed by flamers of Tzeentch. Bit of a mixed bag debut, really.
6th and 7th Edition
A bit foolish for Forge World to debut a model so late in 5th, literally months before 6th edition hit. Sometime later, the decimator had an update to accommodate the existence of Hull Points and close combat weapon AP values. The Decimator's armour was vastly improved to 13/12/11 and it received 3 hull points, fairly standard for Daemon engines. Siege claws became S8 AP2 and shred that could do a smash attack and once again, heavy flamer the inhabitants. The rest of the weapons stay the same although by this time, the butcher cannons had become redundant as the CSM debuted the Forgefiend with the then much better Hades Autocannon. Vehicle dedication also slightly changed: Khorne got Rampage, Nurgle got It Will Not Die and Slaanesh and Tzeentch were unchanged. On top of all these changes, the Decimator got a 40 point base price increase.
No significant changes to individual usefulness but the unit was no longer underwhelming - rather it had some sort of purpose and that's pretty neat.
8th Edition
A major change over, the decimator now is useful in melee largely against hordes and very proficient at range! Butcher cannons now offer a debuff which is okay, storm lasers are largely the same, the conversion beamer is still there and then there's the soul burner petard, which now straight up hoses mortal wounds on things - fucking savage! Arguably now a better, more survivable gun platform now even though it gained an improvement to hit in close combat. Thanks to the vigilus book and the new units suchas the lord discordant, it now gained a lot of sinergy.