Fun concept here with a very tight and usable little sewer area!
A few favorite things:
- Fun voice and useful boxed text to introduce the rooms
- The ratfolk demonstrating the potion (if undisturbed)
- The Elixir of Destructive Egress is a very fun idea
A few specific thoughts. Kospharnos's seems to have conflicting motivations. His aim is either to send the party in to recover a stolen elixir/discover the results, or to have them unwittingly become new test subjects. It reads like he's trying to do both, which doesn't make sense to me.
The elixir's effects could be simpler. 1d4 + 2d4 rounds of delay is arbitrarily long and complicates the ratfolk demonstration. If the delay is part of the fun (which it is), keep 1d4. Likewise, 3d20 damage and another 3d20 damage if they aren't dead? Just say: they explode and...are therefore dead (similar to "...as is proven by the fact that he is now a corpse." :)).
"Until I perfect the coloring, I can't sell them." An excellent ruse!