you need to hang around in and out of the succubus tower, eventually rinny won't be in the base of the tower, then you head up to the dungeon and an event should trigger. might have to do it multiple times, but then cassie will mention alissa isn't coming home, and you start an intervention
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I was mostly sleeping inside the tower, and I don't know what specifically triggers it - it's a random event so you just have to pass time in the right way for it but I d on't know what's the right way. keep trying. Oh, also make sure you already have started the hypnosis training for alissa (poster outside the inn), maybe experience it once yourself for good measure