The Prison of the Planes/Skill DC

Carum Imer: "Did you find it yet? Do you have the relic?"

You: "No, but I did find a locked door. Grim Goodbellows said you took the key..."

Carum Imer: "Oh yes, well, the gold key. I just might have found it, but why would I want to give it to you? It's obviously important, and if I find the relic myself, riches unimaginable will be mine."