Here's the situation: A fire titan on his own demi-plane sent the party on a side quest to retrieve something for him and gave them the rune combination for a teleportation circle that would get them back to his throne room. I want the titan to betray them by having the circle resolve in mid air over a pit of lava so they would fall towards it.
The Create Teleportation Circle ritual will probably be the basis for creating the circle, and the players will probably use Planar Portal to get there. But the reason it's a little tricky for me is that we're in the Elemental Chaos here and I can reasonably assume that the titan can more or less will whatever the hell he wants in the demi-plane, so I don't know how constrained I necessarily am to the Create Teleportation Circle ritual.
Is there anything in the rules that would prevent me from doing this? Must a teleportation circle resolve on a surface instead of mid-air?