If all units at a battlefield die in combat and there is still a hidden unit on that battlefield, can the hidden unit be activated after showdown resolution, or does it trash?

Ruling: If combat has fully resolved with no deathknell or other triggers/abilities remaining, you cannot activate the hidden unit. The hidden unit will trash.

Nuances:

• If there is a deathknell trigger from a unit that died, you can activate the hidden unit during the cleanup step (specifically between the special cleanup and remove control steps) before the cleanup step ends.