Protect the Crystals From the Goblin Hordes! is a Phase Beast Challenge encountered in Frosted Hills. There are five large crystals placed on two levels, which get attacked by goblins. Saving a crystal from destruction yields a chest as a reward each.
After enemies are killed around the first crystal (if saved the reward is one small chest), a gate opens and a portal leads to the level below where four crystals offer one medium chest each, if successfully protected.
This Phase Beast Challenge is (in NG+) the best location to farm for Skull of Limoany (All Damage Taken is reduced by 5%).
* crystal level reflects the fact that the image is from an NG+ playthrough