Yagrum Bagarn is a resident of the Corprusarium, and the last known living Dwemer. He is infected with Corprus, and can only move around with the help of a modified Centurion Spider's lower half. He is a good friend of Divayth Fyr, a powerful, ancient mage who owns the Corprusarium and Tel Fyr. Yagrum is also the author of Tamrielic Lore.
Bagarn plays a part in the Morrowind main quest (the Nerevarine must retrieve a pair of boots he is trying to repair for Fyr). Later, if necessary, he can activate the Wraithguard if the Nerevarine has a reputation of 20 and Bagarn has a disposition of 90, or just a reputation of 30, or the documents "Dagoth Ur's Plans" and "The Plan to Defeat Dagoth Ur" (which are in the same room as Vivec—thus easier to obtain than the non-working Wraithguard). He will also need Kagrenac's Planbook and Kagrenac's Journals, since he is not an expert on the Wraithguard's use (although he helped create it). Once the requirements are fulfilled, the Nerevarine will need to equip the Wraithguard (and will be hit for 201-225 damage the first time it is equipped).
He can also play an important role in the "Disappearance of the Dwarves" quest, because he is one of only two people in Vvardenfell who can readily translate Aldmeris (the other being Baladas Demnevanni). After translating the books The Egg of Time and Divine Metaphysics, Bagarn says that he would ordinarily explain in detail the theories presented in the books, but believes that Kagrenac's ideas must die as their master has.