Clivet is a member of the Cult of Hazeel. While doing the Hazeel Cult quest a player must talk to him to choose sides. Should a player choose Hazeel's side, they would get a Mark of Hazeel. Though he's level 13, he's immune to poison.
However, if they choose the Carnillean side then Clivet flees on a raft in the sewers and they must manipulate the sewer valves to get to him.