World Trees are just that; trees with worlds on their branches. They are not the Yggdrasil, but some believe they are its seeds or shoots. There are theories that our realm is inside of another World Tree, or that the Yggdrasil is a World Tree of unfathomable size that holds all the multiverses on different branches.
Physically, World Trees are massive. The smallest of them are ‘only’ forty or so feet high, while the largest ever recorded on Earth proper reached over three miles in height and just over a mile and a half in diameter at the trunk. At their peak is a massive, lotus-like flower that holds an immense water bubble, inside of which is a stirring microcosm. World Tree microcosms tend to contain singular worlds and the celestial bodies needed to promote the formation of life, but elder trees can have whole solar systems or even small galaxies, sometimes even growing secondary microcosms on separate growths. The World Tree with the most recorded microcosms topped out at six.
Journeys into these cosmos’ are possible with the trees permission. Beginning no more intelligent than a rodent, as it grows, the World Tree becomes significantly smarter, wiser, and more observant, while gaining considerable magical power focusing on the creation, alteration, or relocation of matter. Despite their immense power, they’re either unable or unwilling to actually interact with their internal worlds in meaningful ways. On occasion they may wield their power to stop some destructive event or whisk some malefactor on their world away before it can do harm, but otherwise they merely passively observe their creation, and the world around them.
Despite the curious beauty they have when fully grown and the wonder that comes with exploring their alien worlds, the World Trees are not exactly a welcome sight everywhere. The energy costs associated with creating their microcosm is not insignificant, with many civilizations viewing them as plagues because they drain the very life from the soil as they grow and darken the skies as they take their lions share of sunlight to fuel the formation of their worlds. They’re technically classified as planetary parasites, but the essence they drain from their surroundings eventually stabilizes after a few centuries until the tree and the area around it achieve an equilibrium. The tree’s own arcane power allows it to give back everything it has taken, and the strange energy that pours out of its microcosm causes life in all directions to flourish. Not everyone has the patience or desire to wait the three or four hundred years this process needs to begin breaking even, and dozens of worlds that could have been were prematurely destroyed before the World Tree even began to flower.