Are the clerics willing to make such gross violations of their word, given their alignments and dieties? Have they considered the ramifications, both in the current, and afterlife, should they fail?
It might be worth dropping a hint or two just as the groups are about to meet, about the magic dripping off the drow. Unless, of course, the drow are now bent on wiping out these annoyances. In which case, they've deserved it. They made an agreement with the drow, and are now backing out because "they're tired of making deals."
You might want to have the drow make a strong show of arms for a round or two, and then leave. Maybe concentrate on a PC for a round, drop him, and issue a warning to the rest of the party, as they plane shift or mass teleport or something out of there. Use the escape as a further show of power - using a high level spell.