Azrael saw a flicker of reflection - an immensely bloated daemonic creature with a dozen fanged maws and a thousand eyes. Yet beyond the daemon, inside its immaterial form, he saw a vast worm, coiled about the core of the daemon, feeding on it's own tail.
"They made us, the one split into three,’ the essence of Chaos, refined and shaped. They thought they could tame the warp, use me to dig their tunnels and secret ways hidden from the eyes of the Powers That Rule. They did not know that they made something else. Something far grander."
- – Tuchulcha on itself
The Tuchulcha Engine used by the Lion during the Horus Heresy is a truly interesting entity. It is an incredibly ancient sentient device of unknown origins; though it does mention that it was created by beings that used it to "tunnel their secret ways hidden from the eyes of the powers that rule", which sounds suspiciously like the Old Ones and their creation of the Webway (and on a metatextual level, like a TARDIS, but we'll get to that later). If its creators are indeed the Old Ones and it was indeed used in the creation of the Webway, then it is possibly the oldest thing still around by the time of 40k, and may even predate the Chaos Gods themselves seeing as they came into being during the War in Heaven. (Warp entities existed before the Ruinous Powers existed. As linear time doesn't apply consistently in the warp, this may or not be true; it's all so very Chaotic, isn't it?)
Tuchulcha claims that it cannot be destroyed as it exists in the past, present and future simultaneously, and that if it was ever destroyed then it will no longer exist in any time line, though it admits that the Lion scares it as it has no wish to die.
This brings up an interesting dilemma that the guys at GW could not possibly have missed, as it turns Tuchulcha into possibly the ultimate reset button; if Tuchulcha was indeed used in the creation of the Webway and if destroying it retroactively erases it from reality, that means if it does get destroyed then it couldn't have been used to make the Webway. And if the Webway doesn’t exist, then the Old Ones and the Necrontyr may never have crossed paths, which means that the War in Heaven never happens. The Eldar and Orks are never created, the C’tan are never discovered, and the Necrontyr never become the Necrons. With the War in Heaven never happening, the turbulence that it created in the Warp never happens, and the Chaos Gods never come into being. Without them the ancient Shamans never sacrifice themselves in order to protect their souls, the God Emperor is never created...and so on.
TL;DR: if Tuchulcha is ever destroyed, the resulting chain reaction of time paradoxes will erase the 40k universe as we know it and humanity would safely rise and eventually become New Man like the Emperor wanted (probably to the Old One's joy). C'mon Lion, kill the fucker! Though that would also erase the grimderp and half of the grimdark. So we can't have that.
Tuchulcha is part of a trinity of sentient Warp constructs consisting of itself, the Plagueheart and the Consumer (the creature at the heart of Caliban, also called the Ouroboros):
- Tuchulcha takes the form of a large perfect sphere, presently hidden deep within the Rock and guarded by the Watchers in the Dark.
- The Ouroboros (also known as "The Consumer" or "devourer worm") is a gigantic nightmare creature almost the size of a planet that comes straight out of H. P. Lovecraft. Its current state is unknown, but it is likely still hidden within the ruins of Caliban. During the event of Unforgiven by Gav Thorpe, the Dark Angels and Cypher attempted to slay it, but they discovered that its "heart" was somewhere else.
- The Plagueheart is a vast living planet like entity that looks like it was the love child of a Great Unclean One and Marvel's Ego the Living Planet, presently in the possession of Typhus and Merir Astelan.
When the three are combined, they can create a rift that bridges space and time. The three entities were apparently at one time part of a single whole, but were split apart at some time in the past by some unknown means. On its own, Tuchulcha is capable of extremely efficient, precise, and accurate Warp jumps by calming or flattening the Warp around it.
For some reason Tuchulcha claims that Caliban is its home and that salvation is waiting there for it and the Dark Angels.
'Who? Who made you?'
'At the dawn of the galaxy, so far removed from humans they might as well be gods. But even they could not tame the warp, only corral it for moments at a time. But that which creates also devours, and i am the foundation of all that was, is and will be. I am the Lens, the Bridge, the Doorway.'
The Great Crusade
Tuchulcha apparently took a great liking to the Lion; it says that as soon as it felt the presence of the Lion, it knew its “saviour” had finally come. Its apparent fondness for the Lion borders on almost stalker-like levels of obsession: although it could have left at any time, it chose to remain in the exact same spot for more than fifty years waiting for the Lion to return and claim it. Even when Typhus turned up wanting to save it, Tuchulcha told him to go away as it would only leave with the Lion.
The Horus Heresy
During the Horus Heresy, the Lion would come into conflict with the Death Guard under Typhon over Tuchulcha. The Lion prevented Calas Typhon from acquiring the engine and was determined to use it to defeat the Night Lords and their Primarch, Konrad Curze. Lion El'Jonson began to frequently converse with Tuchulcha in the aftermath of the battle. Tuchulcha, of course, was overjoyed at the Lion's return and constantly wanted to know how it could help.
The Lion would use Tuchulcha to enter the realm of Ultramar through the Ruinstorm brought on by the devastation of Calth. After joining Guilliman's Imperium Secundus, despite the fact he had only a small fleet and 20,000 warriors with him (the rest remaining on the opposite side of the Ruinstorm with Corswain) he would employ it to great effect against the forces of the Word Bearers and World Eaters who were plaguing the 500 worlds. All the time throughout, Tuchulcha remained fairly reliable and never lied to the Lion, albeit speaking in a cryptic manner.
When the joint visions of Konrad Curze and Sanguinius proved the lie of Imperium Secundus, the three legions set out for Terra. Initially leaving separately, the Lion ordered Tuchulcha to take him to Terra, and was instead delivered to the Pandorax system. The engine later explained that it took the Lion as far as he needed to go, but no further until certain barriers were overcome; it became almost belligerent in its further explanations, saying that the Lion could not go directly to Terra, but could not explain what the barriers were, arguing that it could only see so far and it was not omnipotent.
After picking up some members of the Shattered Legions and getting pointed in the direction of Davin, the Lion called his brothers to him so that he could lead them through the Ruinstorm together - at which point Tuchulcha noped him again and deposited the fleets at a literal brick wall in space, one billions of miles wide and thousands of miles thick, only made possible by being summoned from the Warp and anchored to a conquered Forge World.
The Engine then dropped the fleets in a region of space filled with giant solid blocks, light years in diameter, again summoned from the warp. The three Primarchs realized this was what the galaxy would look like if Chaos achieved its victory. Then Tuchulcha finally took them to Davin, which was surrounded by a boneyard of actual skeletons and ruined buildings, layered millions of miles thick, and it took hours to shoot their way through.
When their business on Davin was completed, the way to Terra was revealed. But the Lion would not go to Terra, because Sanguinius needed a diversion to make it there himself and face his own destiny. So the Lion and Guilliman set about assaulting the Traitor force's rearguard to start drawing them away from Terra and give Sanguinius a chance.
Much later in the 41st Millennium, Cypher revealed that Tuchulcha was embedded within The Rock and that the Dark Angels' dark history of The Fallen could potentially be changed by finding the other two engines and changing history. He also warned that Astelan and Typhus both sought the engines as well, the former trying to ensure his rebellion would be a success (talk about a pipe dream) and the latter attempting to join forces with his past self. Eventually all 3 devices were gathered, and a warp rift was summoned over the ruins of Caliban.
'It is done, the plague-lord will no longer interfere. We will be remade.'
Convinced by Ezekiel to leave history be and to focus on trying to prevent nearby Fallen and Death Guard forces from accessing the rift first, Azrael destroyed the rift leaving the status of the other two constructs unknown, and it also had the unforeseen consequence of +++absolutely nothing of note happening. Any further questions should be forwarded to Interrogator-Chaplain Zacharias and his strange new friends.+++
Yeah, a living machine with a human-like avatar with the ability to travel through time and space, and that is much bigger on the inside than it has any right to be and the ability to open seemingly random doorways inside itself that weren't there before, all centered around a glowing orb power source? The writers don't dwell on it, but this thing is definitely 40k's answer to the TARDIS.