Retrieve the Heartswood from Ashenvale and bring it to Gakin the Darkbinder in the Mage Quarter of Stormwind.
In preparation for your coming, I used my sight to look over the world and through the mists of time. I have found what it is you seek.
In the War of the Ancients, two lovers fell together in battle. Where they took their last breaths, a tree known as the heartswood grew, a symbol of their undying love.
That should be enough to tempt a succubus. You will find it in the ruins of Ordil'Aran in the northwestern part of the forest of Ashenvale.