Skyrim's Hardest Enemy
Karstaag is an undead frost giant found in Skyrim: Dragonborn, hard-capped at level 90 with full immunity to frost, poison, paralysis, most crowd-control effects, and many powerful enchantments. He regenerates health quickly, and his stomp can send you flying. You must summon him yourself at Castle Karstaag after retrieving his skull. Defeating him grants you a rare summon ability usable three times.