User blog comment:MDGeistMD02/Which Witch is Which?/@comment-6301921-20131123142036/@comment-3242762-20131123192650

I'm 99.9% sure I have the solution: It all depends on whether you choke/kill the witch talking to Delilah, nothing else.

I had two different saves right before fitting the engine onto the ship, one with the Delilah witch dead, the other with her alive.

The witch dead = 5x NO attack.

The witch alive = 5x attack.

It's not pre-determined, it's not random, it has nothing to do with the scout, it has nothing to do with chaos, it has nothing to do with being spotted, it has nothing to do with the five sewer witches, (differentiated, here, from the 'Delilah witch'.) it has nothing to do with speaking to Delilah.

Soooo, all you have to do is choke/kill the witch speaking to Delilah to prevent the witch attack on the Undine. (Just to add, if you ignore the room completely, the attack will still take place).

solved.