A daemon engine is, as the name suggests, a machine which has been constructed/fused with a Daemonic entity, resulting in the combination of the two into a living mechanical creature that often is all but unstoppable.
Daemon engines can be found in many shapes and forms in a number of RPGs and miniature games, often representing some of the greatest horrors, and most derpy models, in those settings.
In Warhammer 40,000, daemon engines first appeared in the Specialist Game Epic, a sort of precursor to Apocalypse that used really small models to try and depict mega-huge conflicts. Daemon Engines were the Chaotic counterpart to the various non-Titan Imperial mega-mecha.
After Epic vanished into the mists, daemon engines were of little importance in the Chaos Space Marine codices; for a considerable time, the only equivalent were "Possessed Vehicles", which were vehicles given an upgrade so that they were possessed and controlled by daemons. Proud fa/tg/uys will recall that back in 3rd Edition, there were actually two types of vehicle possession: the first was standard vehicle possession which bestowed an immunity to crew shaken/stunned results, as there was no crew to be shaken; the second was called "parasitic possession" and granted the vehicle the ability to un-immobilize itself or regrow destroyed weapons on a 4+ at the beginning of every turn. Then Gav Thorpe thought Chaos needed to suck so he threw on some stupid fucking downsides. First, he limited which vehicles could be possessed, meaning no more possessed chaos dreadnoughts; take our word for it, they used to own. Second, possessing vehicles reduced BS because apparently possessing a vehicle with high-BS greater daemons or even fucking Bloodletters was totally out of the question for some retarded reason! Traditionally, transports couldn't take this upgrade as the hull had to be sealed in order to hold the Daemon inside it but then Gav thought that he was someone else when writing the codex and decided to totally ignore the canon; this resulted in transports like the Rhino and Land Raider being able to be possessed but at a chance to eat a passenger and sans 1 BS. Finally, 6th Ed's CSM and Daemons codex decided to take another swing at Daemon engines by effectively turning the upgrade into better extra armour (again, at the expense of one ballistic skill) and so what was traditionally an immunity to crew shaken/stunned is now ignore shaken/stunned results on 2+; not bad by any stretch, but inferior to what it replaced.
Right, back to history: the first "proper" daemon engine to grace the fields of Warhammer 40k was the Defiler back in 3rd edition, a giant six-legged crab/spider walker with a big cannon, claws and an intense amount of terror added. It's important to remember that in those days, it counted both as a walker and a monstrous creature and though it had a hilariously weak WS2, it also had full BS4. Such was its popularity that further daemon engines have been added to subsequent codices, allowing a Chaos army in 40K to unleash Defiliers, Maulerfiends, Forgefiends and Heldrakes. With the birth of Apocalypse, many of the older daemon engines have also returned, and new ones have joined their ranks with Forgeworld creating yet even more fuck-awesome Daemon engines! Hell, even some of the Chaos-aligned Titan models are shown to be daemonically possesed; open up the cockpit and you'll see the crew's been assimilated by nasty growths and visibly writhing in agony. That's Chaos, for you.
A current complete listing of daemon engines would be as follows...
- Defiler - The original 40K Daemon Engine, a crab/spider tank created specifically for hosting daemons. Based on the Dreadnought, and brainchild of Abaddon. Appears in the Chaos Space Marine codex. Specific God-aligned variants known as the Slaughterfiend (Khorne), Desecrator (Nurgle), Debaser (Slaanesh) and Deceiver (Tzeentch) have appeared in other sources, mostly the Fanatic Online fan-zine, but were also a thing in 3rd edition when that was allowed (and excellent).
- Soul Grinder - A
CyberdemonDefiler "reforged" in the Warp so the daemon has greater control over the machinery (yet has nearly identical stats to the Defiler, barring the better armour). Highly prized by the daemons themselves, to the point they actually form an independent faction in their own right. Appears in the Chaos Daemons codex.
- Decimator - A sort of daemon-controlled uber-Chaos Dreadnaught; known for being remarkably resilient. A Forgeworld original unit that has yet to appear in any story lines and appears to have been made because Forgeworld just loves randomly cranking out the coolest shit!
- Forgefiend - Hulking bestial daemon engines that specialize in showering enemies with ranged death. Appears in the Chaos Space Marine codex.
- Maulerfiend - The Forgefiend's melee-specialist cousin, specializing in tank-killing and bunker-busting. Appears in the Chaos Space Marine codex.
- Heldrake - A dragon-like daemon-possessed flyer. Appears in the Chaos Space Marine codex.
- Death Wheel - A self-propelled, spike-encrusted siege weapon that basically rams its way through obstacles and over infantry. Appears in the Black Legion e-book supplement for the Chaos Space Marine codex.
- Lord of Battle - A Titan-sized mechanical replica of a Bloodthirster mounted on a giant tank for a lower body. Appeared in Epic 40k.
- Cauldron of Blood - A huge tank/storage tanker fusion that sprays gouts of molten-hot daemonic ichor over foes. Appeared in Epic 40k.
- Tower of Skulls - A tank/cathedral cross featuring huge tower-turrets made from the skulls of slain enemies. Appeared in Epic 40k. Revamped by Forgeworld for Apocalypse.
- Death Dealer - A mechanized siege tower equipped with a mechanical warrior at the front for hitting things with. Appeared in Epic 40k.
- Blood Reaper - A huge mobile gun battery. Appeared in Epic 40k.
- Doom Blaster - A massive tank that sports four super-heavy mortars, used to bombard enemies from afar. Appeared in Epic 40k.
- Brass Scorpion - A daemon engine that resembles a huge mechanical scorpion with a supped-up gatling gun "stinger", a mouth-mounted Demolisher Cannon, two front-mounted "Hellmaw" pattern flame cannons, and a pair of giant mechanical pincers or motorized saws for ripping things up in melee. First appeared in Epic 40k, revamped by Forgeworld for the Siege of Vraks and Apocalypse. Also appears in the Tome of Blood sourcebook for Black Crusade.
- Blood Slaughterer - A daemon engine vaguely resembling a blade-limbed spider or monstrous tick. First appeared as a "Chaos Robot" in the early 2nd edition/late 1st edition. Revamped by Forgeworld for the Siege of Vraks. Also ppears in the Tome of Blood sourcebook for Black Crusade.
- Blood Throne - Basically a chariot for Heralds of Khorne and a damn deadly one at that! Appears in the Chaos Daemons codex.
- Skull Reaper - A legendary Titan-sized daemon engine that resembles a mechanical Bloodthirster with one arm bearing a chain axe and the other being a turbo-melta. Appears in the Tome of Blood sourcebook for Black Crusade.
- Nurgle Plague Tower - A daemon-infested, diseased pus-spewing mechanized siege tower. Appeared in Epic 40k.
- Contagion - A trundling daemon-possessed catapult that launches diseased projectiles. Appeared in Epic 40k.
- Plague Hulk - A Nurglite variant of the Soul Grinder with far more pestilential attacks. A Forgeworld original unit made for the Siege of Vraks.
- Blight Drone - Plague-spreading insectile VTOL hovercrafts. A Forge World original unit made for the Siege of Vraks.
- Hell-Strider - A comparatively small corrupted Imperial Knight that uses a central lascannon and twin melta-beams to hunt down armored foes, relying on speed and mobility to get close, make an attack and escape. Appeared in Epic 40k.
- Hell-Scourge - The largest of the corrupted Daemon Knights of Slaanesh, these daemon engines hunt in packs using bristling bolters and their powerful Castigator cannon to scythe down whatever gets in their way. Appeared in Epic 40k.
- Hell-Knight - Corrupted Imperial Knights that use their speed, mobility and powerful (if short-ranged) Thermal Lance to slay enemy heavy armor. Appeared in Epic 40k.
- Slaanesh Subjugator - A corrupted Scout Titan that uses its daemonic Hell Claws, lascannon and brain-scrambling Psi-Pulse to suppress enemy units so other Chaos forces can advance behind them. First appeared in Epic 40k, received a revamp for Apocalypse and also appears in the Tome of Excess sourcebook for Black Crusade.
- Questor - A heavier corrupted Scout Titan that specializes in ranged firepower, most iconic being its rapid-fire Tormentor cannon. Appeared in Epic 40k.
- Fire Lord of Tzeentch - A daemon-possessed super-heavy assault aircraft that used lascannons, Flame Cannons and firestorm bombs to devastate opponents. Appeared in Epic 40k.
- Doom Wing - A daemon-possessed aircraft that uses a hellfire-spewing Flame Cannon to burn down enemy fliers and infantry alike. Appeared in Epic 40k.
- Silver Tower of Tzeentch - Castles ripped out of the ground and carried along by the sorcery of daemons and sorcerer-cabals and used to rain psychic death on anyone below them. Appeared in Epic 40k.
- Aether Ray - A daemon engine that resembles a mechanical manta ray, created by the sorcerer-technocrats of Q'sal as a means of personal transport. Essentially an artificial Disk of Tzeentch that can be big enough to carry squads instead of one person. Appears in the Tome of Fate sourcebook for Black Crusade.
- Mirrorfiend - A giant beetle-like daemon engine containing a raging Greater Daemon of Tzeentch that uses powerful psychic abilities, including an aura that makes viewers see their worst fears and darkest memories reflected in its shining surface, to kill those who oppose it. Appears in the Tome of Fate sourcebook for Black Crusade. You wish there was a model/rules for this fucker!
- The Auruntaur - A unique daemon engine that resembles a monstrous, cyclopean Soul Grinder, created on Q'sal but impossible to control. Appears in the Tome of Fate sourcebook for Black Crusade.
Daemon Engines were one of the few occasions that 40k preceded Fantasy. Although Daemon-possessed armor existed, warmachines having a spirit was something added in later editions. In Warhammer Fantasy Battle, the Chaos Dwarfs, always such cheery fellows, decided to get in on the action and create their own hell cannon, which is powered by feeding people into the maw of the cannon which then fires their spiritual remains as powerful barrages. For a while the chaos dwarfs that came with the cannon as crew were the only official ones made by Games Workshop. Thankfully, Forge World have now made a whole load of chaos dwarfs models so you field an army of the tusky psychos and whats more they have a whole host of daemon engines now, including a daemon train powered by death to pull all of your engines into battle. All hail the rape train of dwarfy death!
The Skull Cannons, and dual Slaaneshi Chariots represent the Daemon side of things.