Tuchulcha

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.

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. 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 time works strangely 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 possible have missed as it turns Tuchulcha into possibly the ultimate reset button; if Tuchulcha was indeed used in the creation of the Webway and it simple no longer existed then the Webway no longer exists 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 and 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 and without them the ancient Shamans never sacrifice themselves in order to protect their souls and there for the God Emperor is never created. In short if Tuchulcha is ever destroyed then so is the 40k universe as we know it.

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):

1. Tuchulcha takes the form of a large perfect sphere. Presently hidden deep within the Rock and guarded by the Watchers in the Dark.
2. 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. Current state unknown but 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.
3. The Plagueheart is a vast living planet like entity that looks like it was the love child of a Great Unclean One and Ego the Living Planet from the Marvel universe. 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

During the great crusade the Dark Angels and the Death Guard bought to compliance the Perditus System were Tuchulcha was being worshiped as a god by the local population.

Tuchulcha apparently took a great liking to the Lion as it says that as soon as it felt the presence of the Lion then it knew its “saviour” had finally come. Its apparent fondness for the Lion borders 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.

40k

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. 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 ***Error*** records cannot be found, there is nothing to see here move along***.