Reverse Dream of the Menasphere

You live a peaceful life alongside your sister Mea, getting by as an independent magic researcher. One day your home is visited by a strange girl, Lirili, who introduces herself as a succubus. You've never met a succubus, let alone any other monster that could talk, but she warns you of a coming monster invasion. Despite being a monster herself, Lirili is adamant that she wants to put a stop to the invasion, and asks for your help destroying the crystals that will facilitate it. You're not sure why, but you find yourself convinced by her story. You set out on the quest with your sister and your new companion beside you.