Shade of Parieth - The person that used key to open portal must speak to him to start the spawns. If the key dropper died or did not make the portal, the key is wasted and the spawns cannot be started without using another key.
Lord Cynreft Mhoire - NPC appears in middle of room after final wave of creatures is killed. Speak to receive a key to the chest. He will also give you a new Jester Prison Key if you have done started or completed The Jester.
Surface Portal - Drops in a bunker outside in Graveyard, there is no door so it's easy to accidentally hit the portal. There's no way back in without using a new key.
Images
Two identical statues that open portal when key is used.
The NPC that appears and gives reward when spoken to.