What does finishing the Dead Island DLC unlock?

Well, I managed to finish the Dead Island DLC last night. The last mission is a pseudo-boss level where you need to take down a Steel Zombie. The reward is 5 gems (similar to the previous mission), and there is no replay reward or any main reward for finishing all 50 missions in the DLC. What a waste...

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Mission 50 before completing the level:

Mission 50

Mission 50 after completing the level:

Mission 50 Complete