There is a comma missing after "However" in the challenge description of Revenge.
Travel to an exceptionally harsh dimension filled with vengeful creatures, including the Trimps. Enemies have 10x health on even zone numbers. If your army is killed at any point, any overkill damage will be applied 750% to the next group of Trimps to fight. Any time a group of Trimps is killed by this Overkill damage, your Trimps gain a stack of 'Revenge', increasing their Attack and Health by 20% (additive). However if your Trimps ever reach 20 stacks of Revenge, you will instantly fail the Challenge. Clearing Zone 80 with less than 20 stacks of Revenge will complete this Challenge.
There is a comma missing after "However" in the challenge description of Revenge.