Mistresses pre Oct 2020 update shows the Mistresses-format before the 26 October 2020 update
Mistress summons give you five chests containing rare, epic, or legendary cards.
A free summons is available every 15 days.
After summoning, whether free or not, the cost of the next summons is discounted, up to a point. If you exit the summons screen, shop or game, the discount can be lost. Prices for the first, second, and third-and-subsequent summons are as follows:
Rewards
The chance of getting a card rarity for each chest follows, according to the 16 April 2019 Update:
(*) at least one legendary is granted with every 10x summons. Should it not happen, the player can contact support specifying the time and date of the summons. Starting from league 5 you have a chance to get 2 stacks of legendaries instead of 1.
Results
The number of cards per chest depends on your max league (not current league).
It is unknown if the number of cards awarded is uniformly distributed in the above ranges.