Sellassars was a male Kendarayan, born in the late 21st century on the planet Kendaray in the Gamma Quadrant. Sellassars was a Dream Rider, one of a small number of Kendarayans who were able to bring themselves to a near-death state, and to move back and forth between the barrier of life and death.
In 2372, Sellassars was sent as an envoy from Kendaray to meet with representatives of the United Federation of Planets at a summit on Denobula Triaxa. En route there from Starbase 375 aboard the USS Enterprise-E, Sellassars inadvertently caused a rash of comas among the crew, due to his unique abilities as a Dream Rider. (TNG - Slings and Arrows eBook: That Sleep of Death)