To open the pillar you need a source point. It tells you as much in the dialogue - it says "you feel like an empty vessel" or something similar.
As there is no source puddles or source points from purging wands any more you need to absorb contents of a soul jar.
The best (in terms of not losing XP) would be one of the 3 skeletons - i.e. not the Jester or Granita. Or Withemore.
It isn't worth it though really imo.
