If the wights are engaged in combat in the Spawn Vats area, roll a d6 at the end of every round. If the resulting ooze is still in a vat, it escapes and tries to attack the nearest creatures.
d6
Ooze
1-2
Black pudding
3
Gray ooze
4-6
Ochre jelly
Citation
Tales from the Yawning Portal Chapter 5: Dead in Thay