Dagoth Gares
Dagoth Gares is a minister of the Sixth House who can be found at the shrine of Ilunibi, provided you are on the related quest. When you first meet him, he will greet you with a message from Dagoth Ur, and then attack.
When you kill him, you will be automatically infected with Corprus. This is part of the Main Quest and is completely unavoidable.
"The Sixth House greets you, Lord Nerevar. Or %PCName, as you call yourself. I am known as Dagoth Gares, priest of Ilunibi Shrine, and minister to Sixth House servants. My Lord, Dagoth Ur, has informed me of your coming. I wish that this time you had come to honor your Lord's friendship, not to betray it."