If my Ekko is killed by Bullet Time and has a hidden Teemo underneath, is there a priority window to play the hidden Teemo before it's discarded during cleanup, since Ekko's deathknell trigger wants to go on the chain?

Ruling: Once Ekko's deathknell triggers, you have already lost control of the battlefield and therefore the hidden card will be discarded. You would have to use the hidden card in reaction to your opponent's Bullet Time before the damage resolves.

Nuances:

• If you play the Teemo in reaction to Bullet Time, the Teemo will also die from the Bullet Time damage since the damage has not yet resolved