Jyrik Gauldurson is a draugr 'mini-boss' encountered deep below Saarthal during the quest Under Saarthal, which is a part of the quest line for the College of Winterhold. He is the primary antagonist of the quest Forbidden Legend and a minor antagonist of the College of Winterhold questline.
Jyrik was the eldest son of Lord Gauldur, the Arch-Mage of the College of Winterhold. He and his brothers, Mikrul and Sigdis, coveted their father's amulet, which held immense power. Recovering the artifact, he and his brothers stormed the cities of Skyrim with summoned armies, ending many lives in their mad conquest. Sent by Arch-Mage Geirmund, a battalion of battlemages was dispatched to end the spree, sealing the brothers away in their respective tombs with Writs of Sealing. 
When his chamber is first entered, Jyrik is invincible. A few seconds later, Tolfdir tells the Dragonborn to keep him busy, and rushes downstairs. He runs past Jyrik and begins shooting bolts of lightning at the orb, draining it of some of its power, which lowers Jyrik's defenses and leaves him vulnerable. His body will glow in the corresponding elements (Fire, Lightning, and Frost), meaning he has increased resistance against them.
Bringing the Gauldur Amulet Fragments to Reachwater Rock, the Dragonborn faces all three sons of Gauldur in battle, one last time. Jyrik is the third brother faced. He wields an Ancient Nord War Axe, casting Flame Cloak on himself. He teleports across the room to evade close combat and will use Unrelenting Force during the fight.
- The Staff of Jyrik Gauldurson.
- The Gauldur Amulet Fragment (which is needed for the Forbidden Legend quest).
- The Writ of Sealing, a small note that reads:
"Be bound here, Jyrik, murderer and betrayer. Condemned by your crimes against realm and lord. May your name and deeds be forgotten forever. And the charm which you bear be sealed by our ward."
Reading this note will automatically accept the quest "Forbidden Legend."