The Elder is the only boss enemy that can be summoned by obtaining eight ancient seeds and burning them at sacrificial altar in the deep forest. The altar is the quadratic stone monument with the fire bowl in the middle, it has a rune stone also, that reads: Burn their young.
He has 2 attacks, one being a stomp, but if you get farther away from him he will throw vines at you. The Elder is about 3 times taller than the player and deals a lot of damage to trees and structures but fairly endurable blows against the player.
Still it is recommended, to bring a stack of health potion to the battle, the 100 HP need to be refilled while you fight the Elder.