The Clown was a sadistic figure born from the fears of sleeping aliens, ruling a virtual world that held Harry Kim and B'Elanna Torres captive.
The Clown was a personification of fear that emerged in a computer system built to keep survivors of an ecological disaster in stasis. Meant to provide a pleasant dream, it grew on their fears instead, giving rise to a carnival master with power of life and death over his prisoners. Michael McKean played him.
In The Thaw, Harry Kim and B'Elanna Torres connected to the system to free the sleepers and became trapped. The Clown refused to let anyone leave, since his existence depended on them. Using a subtle hint from one of the captives, Janeway outmaneuvered him so the crew could shut the system down and free everyone.