Yggdrasil

It is unknown how Yggdrasil actually looks like. The Dark Lords and the most powerful of their servitors appear significantly more aligned with the human minds - with, for example, Yggdrasil and its servants were 'natural' in theme - however it is not their ACTUAL look. It's just a self-censorship that human minds enforce upon themselves, seeing them as the closest approximation that they could handle.   Yggdrasil appeared as a massive tree with a size that made the already impossibly vast trees of Alfheim appear insignificant. It was so massive that its actual size is unknown - what is known is that the base of the tree was three thousand kilometers in diameter, and according to some records, it 'held the stars of Alfheim captive in its branches'. But its actual existence was even more vast, for his roots were spread throughout the multiverse, sapping all life from the soil to satiate Yggdrasil's endless hunger.

Yggdrasil commanded legions of his spawn-servitors, and possessed numerous potent plant-themed powers. It conjured storms of pollen that could take roots in a living being and develop into a plant-like parasite capable of controlling the victim's body from the inside (and, if killed or reaching the ends of its lifespan, it would make the victim bloom - spreading even more pollen). Its roots could erupt from the ground, impaling everyone in the area and devouring them whole. It's bark appared to be impenetrable, with even the most potent of Mankind's superweapons failing to made a dent - but it was eventually ruled out as almost instant regeneration, for as long as Yggdrasil could suck life from other worlds.

Yggdrasil was an invasive plant to end all invasive plants. It grew to an impossible size by lodging his roots throughout numerous planes of existence throughout the multiverse, sapping all life from them in the process. The only reason why it failed to change the entire multiverse into a burned out husks was because other Dark Lords kept it at bay. Due to that, it was actually the first of the Dark Lords to be attacked, for if the others were slain first, it would grew unchecked.   Mankind's tactic employed against it was rather simple, if well-thought. Yggdrasil's connection to other worlds was severed with a deployment of a form of reverse Reality Protection Array, not only stopping everyone from leaving but also stopping the attempts to enter the world from an outside. Due to that, Yggdrasil could no longer drain other worlds - and when in weakened state, Mankind's forces already in Alfheim began weeks long campaing of ceaseless bombardment, deploying the most powerful defoliants known to their species in quantities that would have killed an ecosystem of numerous planets.   Once Yggdrasil's branches began to wilt, the assault was switched to more conventional form, with thousands of nuclear and meta warheads directed at it, together with majority of Universal Assault Units possessed by Mankind at this point. With the armies of Yggdrasil mostly depleted following desperate attacks against the RPAs-holding citadels, the resistance was swiftly overcame, and the Yggdrasil burned to the ground in a fire that lasted for months.   It is believed that when cornered it has managed to spread its seeds throughout the world, at least considering Alfheim's swift change from desolate wasteland to a vibrant paradise of kilometers-long trees.