The first completion of a Season III stage, on both difficulties, awards the player with Valhalla Coins. Valhalla Coins can be used for free summons in the new Valhalla Summon. You need 100 Valhalla Coins for one Valhalla Summon.
Please see our article What is the Valhalla Summon? to find out more about this Summon!
For further gameplay help and discussion regarding Season III, we highly recommend visiting our Empires & Puzzles Community Forum.