Yggdrasil Tree can be obtained by having Cu Chulainn (Caster) reach Bond Level 10.
Yggdrasil Tree is an exclusive CE to Cu Chulainn (Caster).
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
Dragon Fang
Gandr
Ereshkigal
Arthur Pendragon(Prototype)
Gorgon
Black Holy Grail (The Black Grail)