So here is the boss ! He might be stronger than vanquish the overseer.
The boss could be summon hardly but to find him you need to find this npc on the surface. (he's hard to find and spawn really not often)
He'll ask you about him being a multi-world traveler, he wants to fight you at start of the night.
Like the cursed guy at the dungeon, when night happens you can talk to him and make him become the boss.
The boss has specifics mechanics that makes him hard to kill.
First : He summons a lot, i've actualy 4 units but he should summons at least 7 different units. Here i'll show only one to don't spoil all the powers of this boss.
This one is a nearly immune to knockback unit that go faster with the time (like the unicorn or the frost wolf) but doesn't have slow time when you do knockbak to him, he slow to the basic movement speed and restart the get faster.
Next : The boss always gives you a debuff that dishallow you to use wings and flying mounts !
You're stuck on the ground with his summons !
And the worst : With using spells to kill you, he also can heal himself a lot, by one of his spells and when any of his summons deals damages to you he heal himself more than they deals damages.
The boss will have 3 phase in the fight.