Lumian still has 3 contracts, all 3 should be chosen wisely. One for mirror world creature, the other two should be something that helps him in battle and creature doesn’t demand too much in return.
I'm thinking something like a danger sense, something to intimidate like the old Emperor mark, something to enhance his attacks further, something to give him more endurance (physical and spiritual).
A Mirror World creature might give him the shattering Mirror defensive move. Which would be nice. Other than that, Evernight's sleep and concealment abilities would be fun to have. A Demon's Danger sense is also an awesome help. Maybe some type of purification would be cool.