Here is another replay that the fatigue damage mechanic came into play. It does seem like the rule is always after round 20 all cards will begin to take damage directly to their health. This time luckily

Was doing my daily challenge using an Earth summoner. Love the unicorn because it has a huge amount of health and thought I had this win in the bag. Then I learned about fatigue. The "Divine Healer"

Fairly new to the game but I have started favoriting Water. I think magic is the best attack type since it's harder to defend. Would really like to see this game take off!