There should be no special reward at all for 10 skulls. The reward should be that the quests/dificulty are so hard and well designed that the mere fact of getting a completion on such dificulty is the reward itself, the joy of having archieved something so hard earned and dificult to do while having had a good time doing it.
If you put meaningful rewards, even if it's a mere +1 spell power, ppl will run it just for the rewards even if they don't enjoy such challenges. I would and no special reward at all to avoid that. Loot runs and grind should be a thing of lower dif settings which are also more time-efficient, lets leave 10 skulls just for the fun and the challenge.