Koshei the Deathless

When you enter in his room he will respawn if you haven't defeated him yet. He can't die unless you know what to do. If you kill him, he will respawn again. Gives no loot or experience.
This book and its sequel give an overview of Koshei's history.

Will try to kill you while summoning a Bonebeast.

To all vocations: The use of Garlic Necklace is recommended. Even though he has many hit points, he is quite easy to defeat with a 45+ character. His self healing is really troublesome (you could end up doing 8000 damage while Koshei heals 5000 hp, before you defeat him).
Mages should stay away and using their wand/rod and Exori flam or GFB runes since fire is Koshei's weakness. He sometimes does high damage so Mana shield is recommended for Mages around level 45.
Paladins stay away while shooting bolts / arrows / throwing weapons and Exori San (Holy Damage). Some Health Potions would be good. Make sure to have enough mana for Exura, as the Bonebeast can Paralyze you.
Knights will have a very hard time with this one, as he has many life draining attacks, coupled with the fast healing. Not recommended for Knights under level 70 (Fierce Berserk), unless you are experienced. It's also recommended to bring a shooter to help with the killing. As a knight a 20-40 Mana Potions and a few Strong Health Potions are needed as he hits hard. Also a good idea to wear a Garlic Necklace, as it has many Life Drain attacks.
Note: that he will respawn shortly after you've killed him, unless you know what to do. Part of the Koshei The Deathless Quest. Can only be successfully defeated if you use the Koshei's Ancient Amulet on his corpse.