#lifetree

The land-dwelling species of Ocaelum didn't use to live under Lifetrees, but the "Old World" they come from is misleading, it isn't another plane. They originate from the surface of Ocaelum, back when it was a thing. As far as how that happened, one would need to go back very far...

To the genesis of Ocaelum. The guide sometimes refers to the "sun below", the one at the center of the world that provides its warmth to the core and the lower waters, from which the Elder Smiths were born and draw their power from. Ocaelum also has a second sun, the sun above, that provides warmth and light to the surface world, much closer to what people are used to from other planes. The Depths were actually the part in-between, isolated from both suns' warmth and light.

Similarly to the sun below, Elder Dragons were born from the sun above, five of them, complementary to the Elder Smiths, their cousins. They cared for the surface world like the Smiths do for the core, and they might have grown the Elves of Ocaelum like the Smiths forged the dwarves. Back then, the Core extended through the land all the way up to the surface, and even though the Smiths themselves stuck closer to the sun below, they exchanged and had respectful relationship through elves and dwarves with their cousins of the air.

Until one died. Five powerful merfolks from the depths forged an uneasy alliance and managed to kill one of the Elder Dragons of the surface. Those five necromancers then raised the corpse under their thrall, and then, through it, started to steal power from the sun above and split it between each other. Those would use that power to become the Shiver of Kings, unifying all the Depths under their rule and forming an Empire. Despite their many differences over the centuries, they still need each other. One of them dying would release their part of the sun, and none of them know if they could hold to their part if that happened.

As the Shiver killed their brethren and stole the sun, the Elder Dragons felt it, of course. But not soon enough that they wouldn't be significantly weakened by the time they'd find and try to stop the Shiver. Or at least, that wouldn't be likely. So they did their best to save what they could of the surface, by entrusting it to their cousins and the sun below.

There was an exodus through the core and magic both, as the Elder Dragons created the first lifeseeds, and then poured the remainder of their life into them to grow them into the first Lifetrees in a timely manner. The four Councils, or types of Lifetrees are named after (part of the name of) the four living Elder Dragons that created them. The light of each lifetree contains, within it, a faint echo of the sun above.

The surface is now completely unlivable. Walled off by a few kilometers of ice at the end of the Depths and that cover any remainder there might have been of the world it once was. There's barely an atmosphere left, and it is definitely not a breathable one. The magic and leylines up there are long dead. Planeswalking there would trip the same kind of alarms and safety as planeswalking into a wall or in the middle of an active volcano, so as far as planeswalkers are concerned, there isn't a "surface".

Similarly, the upper tunnels of the core, the ones that go through the depths and formed the land above are flooded where they're not frozen and just part of the depths. Without enough warmth to keep alive the fungus, all the entrances let the water in. It is mostly stopped when you get deep enough that the sun below supports the fungal life, but what trickles down forms streams, waterfalls and river in the Core proper, and their sources of water. Spells to make ocean water drinkable are very, very, very common on Ocaelum, it's how all factions that need it get access to it.

The Shiver was too busy stealing the sun, reveling in their new power and conquering their empire to care much about the exodus, the ground waters were too bright and warm for merfolks to care about them anyway.

The Spires themselves know what happened, but elves, humans and coral alike are kept mostly in the dark. Their memories of the surface faded and that might be intentional, to avoid them going on a suicidal campaign against the Vhigg'ithu in vengeance. Some Voices might know the truth.

The Smiths are still very angry at the Shiver for killing one of their own, committing mass murder, stealing the sun and forcing their other cousins to kill themselves to save a little of the surface, and that sentiment is echoed among the dwarves, though very few dwarves know in detail what happened either. The Smiths don't confront the Shiver directly because they aren't sure they can win, with the Shiver having taken power equivalent to their own, and they don't want to risk them stealing the sun below as well should they fail.

The Viggs do as they do, the monarchs very much know what happened but getting information out of a Vhigg'ithu is already hard to begin with, to hell if they are gonna tell anyone how they gained their unmatched power.