That number of vampire spawn in the underdark just felt all wrong. This is what I did
I didn't ascend him but also I killed the spawn. Might seem a bit strange but to me they were both the better choices. Luckily the Gur leader also agreed (Let Astarion do the talking when she turns up in the lair) so I felt good about it overall. I just had visions of the peaceful myconnids being overun by a bunch of vampires with absolutely no self control as they have no experience in excercising such followed by the rest of the countryside outside the underdark - there are plenty of places where they can come back out.
