How do you get yggdrasil?

Teldrasil.

As old as the world I am

In the celestrian king's room, exit through the bg door in the back, then climb the stairs to the top.

To gain the runes (wisdom). Stanza 138: " I know that I hung on a windy tree, nine long nights, wounded with a spear, dedicated to Odin, myself to myself, on that tree which no man knows from where...

there is a yggdrasil leaf at the back of greygnaryl's lair after you have the story part where he dies for upover. none of the other chests have anything in there so it's the only object