Nahkriin, in front of the portal to Sovngarde
Nahkriin is one of the eight high-ranked dragon priests in Skyrim, and is encountered in Skuldafn, a temple you visit during The World-Eater's Eyrie.
Nahkriin, like the other seven dragon priests, has a mask with considerable power, as it gives 50 more magicka points and a 20% reduction on spell cost for Destruction and Restoration spells.
Nahkriin wields a unique looking staff with a powerful Wall of Storms attack. The staff must be used to open the portal to Sovngarde and therefore cannot be kept.
Upon noticing you, Nahkriin will say "Zu'u uth nall [sic] thurri dein daar miiraak."
During combat he will taunt you with the following phrases: "Fent ni filok.", "Hin laas los dii.", and "Sosaal fah hin vothaarn."