Grudge is a way for individual players to fight against each other. It's free to play. Rewards are minimal the winner can plunder but reward worse than single match loss. This is good for testing builds with guild members or friends. Also lets people outside your tier range fight you. Player tier is displayed for people to see and also level. A level 20 player can't join a grudge with someone under level 20. Although a player under level 20 can join players at level 20. Can set a password if your waiting for anyone so random people don't jump in and fight.
Gain 2-6 shards for a win.
Grudge replaced the old Challenge system.