Originally posted by Mercenary
There's no trigger that does that but you can use a func_rotating with the touch_kill spawnflag checked. Use an invisible but solid shader for the "trigger" area. Don't forget to check start_on. I don't know if this works in MP.
Where you want your trigger to be, build instead a brush made of caulk and turn it into a func_rotating. Open up the entity properties [N], and check off the START_ON and TOUCH_KILL boxes. Voila.