The World Tree is a mystical tree central to the plot of Styx: Master of Shadows. It is a colossal tree kept hidden inside the tower of Akenash, guarded by both human and elf alike in a strained alliance. The World Tree possesses a "heart" that acts as the source of the mystical Amber that Styx desires.
Little is known about the history surrounding the World Tree. Presumably at some point it was discovered by the elves who would eventually learn a means of safely extracting the mysterious liquid Amber.