Soul Grinders are Daemon Engines that are essentially the mascots of the Warp Forge, equal parts daemon and machine with the weaknesses of neither (no, that doesn't sound like something from another intellectual property at all), meaning that they can last forever in the material world. They're also one of the few units you can use for a Daemons army that has ranged attacks.
So, if any daemon wants to get to be one of these babies, they have to duke it out with other aspirants near the Forge of Souls in the Warp and prove themselves worthy of becoming a Cyberdemon. The winners get merged to machinery by the Forge's daemonic smiths and must take three oaths before getting let loose:
- All the souls harvested by the Soul Grinder will be used to fuel the Forge. Okay, that's reasonable. They're Daemons, after all.
- The wrecked remains of any war machines destroyed by the Soul Grinder will be offered to the Forge. Well, that's fair considering the gift of near-unstoppability they have.
- Should any of the Chaos Gods attack the Forge of Souls and try to rule it, the Soul Grinder must discard all of their previous loyalties and fight in its defense. Here's where some daemons might cross lines, as this effectively makes them Undivided Daemons. Badass Undivided Daemons unlike those pansy-ass Furies, but still. However, they still have to
buy a Markdedicate themselves to a Chaos God. Presumably that's their day job when the forge is not under attack by a chaos god. Who runs the forge? Nobody knows and there is no official answer perhaps by Malal to convert chaos daemons of other gods. Be'lakor is a possibility as he is one of the most powerful entities of chaos not loyal to a single patron god.
On 40K Tabletop
4th and 5th edition
The Soul Grinder was introduced in 4th edition when the Chaos Space Marines and Daemons were separated into their own garbage codices. It's baseline stats are pretty neat, with F/SA13, two dreadnought close combat weapons, a Harvester Cannon (which on launch was basically a shootier heavy stubber) and a Mawcannon with three optional profiles: "phlegm" or what is basically a battle cannon pie plate, "vomit" or an S+1 heavy flamer and "tongue" - the S10 AP1 shot. Tongue and phlegm had to be bought and former was just not worth it - 1 shot at BS3 was too sketchy. It might occasionally hit something but considering that it had to be deepstruck to get onto the table, you would likely get one shot before the Soul Grinder's fiery demise... which said nothing about the risk in deepstriking this model's HUEG GUTS. You would basically be relying on another squad's icon to guide you or praying a bad scatter doesn't throw your model into the bin. Worst of all was the model's cost, at over 200 pts. base and one of two HS choices in the codex.
5th edition made Soul Grinders even more useless with Wardex Spess Muhreens fucking with deep strikes, blasting them into oblivion and even granting new HS choices--Seeker Chariots specifically--to the Daemons. The Soul Grinder would have to wait until 6th edition in order to get any use.
6th and 7th Edition
With the coming of Fliers, the Grinder has gotten some new use, as its Harvester Cannon (now an autocannon with an extra shot and shorter range) can Skyfire, but doing so disallows the use of it's other guns. It no longer has a Mawcannon, which means you have to buy any ranged weaponry aside from its harvester; the upside is that losing a Mawcannon no longer removes all purchased weapon profiles, so there's that. The old Vomit profile is now called 'Baleful Torrent' and is identical save for gaining torrent, Phlegm Bombardment is Ordnance as opposed to heavy and the Tongue profile is now called 'Warp Gaze.' The dreadnought close combat weapon gets renamed in the new Codex to an "Iron Claw" and additionally, you can give it a Warpsword, striking at the model's basic strength with AP3, but it also grants the grinder another attack, plus a back-up CC weapon. Since 6th Ed, it has to be dedicated as a Daemon to one of the Gods, which comes with specific benefits as listed in the Codex. Being a Daemon of Khorne gives it Furious Charge, Tzeentch lets it re-roll 1s on its 5++ save, Nurgle grants it Shrouded as well and Slaanesh gives it fleet, +3" to running distances and rending.
Tactically speaking, most players prefer to roll with a Nurgle Grinder equipping Phlegm Bombardment with the occasional Torrent and rarely Warp Gaze; the reasoning here is that this shores up the Daemons' lack of ranged support by providing a camping artillery walker with slow and purposeful. Others have found it useful as a close combat walker in dedication to Slaanesh; this build is done with Baleful Torrent and a Warp Sword and is ideal as a support unit in an allied daemons detachment. Let's put it this way, not much can stand up against 6 S10 AP2 rending attacks on the charge, especially not after a scorching from Baleful Torrent; additionally, the Grinder's armour allows it to survive a jog across a battlefield mostly intact. Khorne Grinders had the cheapest point cost (that of the old Soul Grinder cost), making it ideal for players that favor swarm tactics. For the distant few that want a vanilla Soul Grinder for fun, homebrew logic suggests using Khorne Grinder rules while subtracting Mark of Khorne and Furious Charge.
Well things have changed a bit now, haven't they? Now the Soul Grinder has the vehicle keyword, T8, 14 wounds and a 3+ armour save - neato! The downside? It's heavy weapons are at -1 to hit when you move for 5+ which sucks and you want to be on the move to smash things in close combat. Its close combat weapons have more use now instead of Iron Claw backup to somewhat drive home the idea that the Soul Grinder needs to be in close combat.
On WFB Tabletop
It's one of the only artillery weapons Daemons get, and for that, it works pretty well.