What happens is:

You return to Moonrise Towers and everybody is still friendly, congratulating you on a job well done. Depending on your choices with Isabelle, Z'rell will criticize or congratulate you. But no matter what, you will get access to the rooftop where you will meet Ketheric. Again, depending on your choices with Isabelle, he will be more or less happy with you. It's pretty clear though that even his benevolence is just a facade to lure you to the Absolute.

You will then be sent to the altar at the top of the stairs to pray. If you do that, a tentacle will appear and teleport you and your companions away to the Mindflayer Colony right into a bunch of "flesh cells". Here you will meet Balthazar again who laughs at your gullibility and wants to experiment on you. A fight ensues and the rest goes on as usual.