This is one of the sections of the Reward Center.
Screenshot 1-3

You can sign-in once a day. After you accumulate a certain number of sign-ins in a month, you get the following prizes:

Days Accrued Rewards Value (Diamonds)
3 80
22 90
28 200
Mystery Reward 2 Hero Upgrade Chest

Each chest may contain, in turn, one of the Soul Shards needed to upgrade the rank of Heroes. The chest contains only one Shard and the chances of gettting each Shard are:

Important Note: You should claim your reward before the month runs out or your unclaimed reward will be lost.

When you miss a sign in some day, you can have the option to use Diamonds for a late sign in.

Times missed Diamonds
1 2
2 5