I want him locked up in the Prison for all eternity. I want Feyre to visit him once every year, on the anniversary of the end of the war, to share some tidbits of her happy life as High Lady of the Night Court. I want Tamlin to both love and hate these visits, because they are his only glimpses of the outside world, but they are also an annual reminder of everything he has lost because he made all the wrong choices. I want him to slowly, slowly go mad in the solitude, until you would no longer recognise him as the former High Lord of Spring. At that point, Feyre stops visiting him, and from then on his solitude is complete. He is immortal, so his suffering never ends. He remains in the Prison forever.
In my opinion, that is a much more fitting punishment than merely dying.