Lord Kroak

Before the Elves, inheritors to the magic of the world, were even conceived of and before the continent of Ulthuan was raised from the seafloor, before the Dwarfs were even a sparkle in the eyes of the Creators, before mankind first walked the Earth, there was Lord Kroak, Mightiest Toad.

The Legend

Lord Kroak (or Lord Kodak as autocorrect calls him) was the firstborn of the Slann, the first race created by the Old Ones (in their own image) to aid them in shaping the planet to defeat the eventual coming of Chaos. In time, the rest of the Lizardmen were created and served under Kroak. It's possible that Kroak was the one who taught the first Elves magic at the command of the Old Ones, but it could have been any or even all of the Slann.

Kroak did little in the fluff until Chaos first entered the world with the collapse of the Warp Gates, which made the world as accessible to Chaos as a planet sucked into the Eye of Terror. After holding a city-spanning forcefield up for decades during the Great Catastrophe against a literally neverending tide of Daemons, he got fed up of that and turned the field into a titanic explosion that vapourised a hundred thousand of them. As the daemons promptly pulled another infinite horde out of their asses and kept coming, Kroak proceeded to pull his sleeves up and bring out the titanic apocalypse magic that should really have been reserved for the Old Ones themselves. The army book describes it as goddamn time stopping as reality itself struggled to accommodate the tide of bullshit emitting from Kroak's mind, the world around his city folding in on itself to stem the tide of daemons.

They penetrated his defences regardless, and Kroak was ripped to shreds by no less than a dozen Bloodthirsters. Doesn't matter though, as at this point he's too goddamn powerful to give a shit about being dead, and his unstoppable spirit continues to defy them. He basically becomes the singularity at this point, as the world turns blindingly bright and the daemonic horde pretty much just stops existing altogether.

The sacrifice of Kroak is what kept the bulk of the Daemon invasion from facerolling the Elves in one day, and as a result Caledor the Dragontamer had time to set up the Vortex in the center of Ulthuan. Korak's solo efforts freed up the rest of the surviving Slann to channel their own power into Caledor's spell, hastening the weakening of Chaos. Without his efforts the Warhammer World would have been consumed in it's infancy, before man had even left their caves and discovered agriculture.

His spirit lives on to this day, thousands of years down the line, the ghost of his old body now just echoing the titanic destruction he unleashed upon the world. The lizardmen kind of wheel him out on his hoverchair now and then if they need to nuke somebody. The mummy of his favorite Skink rests on his throne, and even in death defends his beloved master (handling all the Close Combat for him).

Kroak's Oracle from the days when he was alive, the Skink Priest Ten-zlati, is the voice of Kroak among the Lizardmen and outranks even young Slann as he knows secrets of the Old Ones not known even to Lord Mazdamundi.

tl;dr Kroak is the God Emperor of Mankind if Emps wasn't a complete and utter failure.