This quest is not available in game.
Using the Heartswood Core, summon and subdue a succubus, then return the Heartswood Core to Gakin the Darkbinder in the Slaughtered Lamb.
Using the heartswood core, you will be able to cast a spell at the summoning circle in the crypt below to open a portal for the succubus to pass through.
Be careful, <name>, succubi have overrun even the strongest of minds.
Once you have bested and returned it to the void, return to me and I will show you how to call and control it.