Defeating it rewards the player with 10,000 candies. According to the game, there is a one in one-hundred chance that a magical seed will become a yggdrasill tree.
Farming the Yggdrasil Tree is not a good idea. because brewing 100 Seed to get at least Yggdrasil once is totally not worth it.
100 Seeds = 65,000 candies. Yggdrasil rewards you with 10,000 candies.