Nope, the Mortal Realms coalesced into being from the remnants of the World That Was. As you said you didn't follow AoS lore, this little video from GW is a handy and quick guide to the Mortal Realms: As to the question of power level, it's important to note that gods in AoS aren't impossibly powerful. Sigmar was unable to withstand the forces of Chaos and had to retreat to Azyr after losing Ghal Maraz. Slaanesh, a Chaos god and unquestionably one of the most powerful beings in existence, was imprisoned by Teclis and Tyrion. Shyish is invaded by Chaos during the Age of Chaos and Nagash is actually slain by Archaon at the Battle of the Black Skies. His mortarchs recover his body and transport it to Stygxx where he regenerates over the course of centuries before he returns and drives Chaos out of Shyish. So the fact that Nagash is a god isn't a de facto indication of power level. Archaon is not a god, and he slew Nagash. Furthermore, when Nagash sees the culmination of his grand plan during the Time of Tribulation, he's not doing it all based around his own power. Nagash has been transporting the realmstone of Shyish from the edge of the Realm, where it's typically found in the greatest concentration, and bringing it to the centre of the Realm in order to invert the flow of magic across all of Shyish. His own magics are what raises up the great pyramids that eventually form the nexus of inversion, but it's quite clear that this is no simple divine act where he can simply snap his fingers and everything falls into place. It's a vast effort brought about primarily by the endless labour of countless numbers of undead workers. Now, in contrast to this, we have Kroak. Little has been written in AoS about the last remaining First Generation Slann, which is a pity since the Seraphon have some very interesting fluff in AoS. But there's some intriguing snippets as to his power in our Battletome: So from this we know that Kroak is the most powerful of those Slann that have died yet clung onto existence. The second quote however is very interesting. What it implies is that the Slann have increased in power since fleeing the World That Was, thanks to being saturated in the power of Azyr during their long journey across the cosmos. Those Slann that have died but remained in existence have become creatures of celestial magic, and it would be a fair assumption that they have a greater level of saturation than a mortal Slann; indeed, Kroak is described as being "filled with the light of Azyr". With this in mind, Kroak's powers have almost certainly increased since leaving the World That Was, and given he was one of the most powerful magic users ever to exist on that world, his powers now would have to be truly immense. That being said, the issue we have is that we simply have no bar to place Kroak against because of the paucity of fluff featuring him. One snippet in a Battletome that describes him as being awesome is hardly enough to form a definitive judgement of how strong he is; Battletomes, Army Books and Codices always present the characters within them as immensely powerful.