The Underworld Tree is where all universes collide. The Tree itself is a large, upside-down sweetgum tree located somewhere outside a galaxy. The location changes based on universe. Can be considered the center of the universe.
There are many ways to destroy the universe or kill a galaxy-deity. The easiest of these would be to locate two Peasant-trees and deprive them of oxygen. This will kill that universe's branch of the Underworld Tree. The next easiest would be to find the Universal Reset Button, out floating in empty space, at the furthest point from any galaxy. Star-carvers guard the Button, which looks just like a normal red button. When pressed, the Universal Reset Button sends the current universe back to 0.0000000001/2^-pi seconds after the Big Bang, essentially restarting the universe. The final and most challenging would be to deal significant damage to the Underworld Tree itself, which will go into healing mode. If damaged again, the tree will die, ending all universes. Shards of Underworld Tree bark can be smelted like metal by the most talented metallurgists or blacksmiths into weapons. These weapons can kill even a galaxy-deity by unravelling them molecularly until they become a miniature nebula of the smallest of elements. The only known weapons in existence are the Stick of Lies, the Underworld Sword, and the Underworld Staff.