Need evaluation of a plan to kill the party

[Batcathat]

I would say the timing is the biggest problem. A trap that takes a day to kill the party leaves a lot of room to survive it. More specifically, I'm wondering how likely it is that the party will spot the giant boulders piled up by the entrance (or are the further away before the party enters? If that's the case the problem might be to quickly get them into position without the party noticing) and how the Decanter is positioned, it seems hard to place it so that it can fill the dungeon without the party being able to mess with it.