In one of the few times where Alex Trebek did not play himself, he got super sinister in a 1996 "X-Files" episode which saw him appear (along with Jesse Ventura!) as a “man in black” menacing Fox Mulder.

https://tvline.com/lists/alex-trebek-dead-best-tv-cameos-cheers-the-x-files-video/alex-trebek-the-x-files/