I don't think he is underrated. He has a relatively big part in the Act 3 story and was the cause that I nearly wiped in my Honour Mode playthrough (where I met him first in game). I had to run from the first fight once and then also only survived the later fight in the underground with his minions because Shadowheart was not immediately involved by pure chance and still could activate the spell to remove curses. The villain so made a big impression to me.
The thought to have him as an endgame ally however has some disturbing vibes. For an evil playthrough maybe?
