There's a chance it clipped under the altar or any boulder there.
I don't know if you have auto-pickup activated or not; but that feature makes it possible to "pick up" items even if they're hidden in a rock or something.

Might worth a try to activate that one first (v-key), do another walk along the area you killed the Elder and if that doesn't work, try to dig down a bit to see if it's hidden under something.
Keep doing that with the auto pickup activated and you will most likely get that key.

Thorin :)

It's all good. I just killed the Elder again with my new easy-kill method (see Elder Easy kill) post.

Good to know. TY

You could use the wishbone to find it, ohhh, yeh sorry, it has to be there somewhere.

Should be right next to the trophy. If you killed him on a slope maybe it slipped down?

Yeah that's what I was thinking too. I searched pretty thoroughly. I guess I just have to keep looking.

Now I'm wondering if I defeated him again if he would drop it again.

He'll drop it again. I always kill bosses twice (one trophy for the stones, one trophy for my house) and they drop all their loot each time.