Trapped succubus

Initiating the quest
If you didn't already, Feed Cassie and make it into her safehouse. On the second floor, there would be a chest containing the key. Take it.

In the western part of the Abandoned Town, there's the closed gate; now, it can be opened. Now, you can enter the Forest Park. The part of park available for exploration is highly limited at first. In the north-eastern part of the park available to you, there's a trapped succubus you can interact with; talk to her.

Freeing succubus
After accepting the quest, an opportunity to cross on the other side opens up, by using the tree to make a makeshift bridge. After that, the only thing required to finish this mini-quest is to go across (your target would be right above the location with trapped succubus) until you reach the cave where some fairies got succubus's tail trapped and now tortures her. You just need to shoo them away. For additional scene later on, you may also tease her some more by yourself.

After rescuing her, just move back and talk with succubus (who would introduce herself as Rinny) again. After finishing the quest, you unlocks new zone – Transylvania.