Very well-done. I love that there are powerful beings that have no problem destroying the party in one action, and impossible for them to kill, being part of the story. That makes the whole thing realistic. Having to deal with problems creatively has really challenged my lovable murderhobos.
My party hated Tobbi, and how wholesome he was. They were gnashing at the bit to simply find the gate and kick the modron through it. The man, who wasn't what he seemed, really threw them for a loop, because they couldn't kill him. They are considering going back to him after completing the quest to find more work. I get the feeling they are an evil group, maybe neutral evil, and willing to work for the highest bidder. Amazing what an RPG brings out in people you think you know. I'm a little disturbed by their responses to the moral quandaries in this adventure.
The ending was poignant, yet could lead to several follow-on adventures. You should totally write a short intro adventure to Sigil, or Mechanus, to add onto this tale.
8/10 with rice. Well done.
[5 of 5 Stars!]