Heldalf has unsealed several Malevolent Crucibles across the continent in order to nurture the growth of strong hellions throughout the land. These arenas attract local hellions and pit them against each other to weed out the weak and leave only the strong. Each of these dark altars can only be entered by a specific party member, alone, or by Sorey or Rose and one accompanying seraph. Examining a Malevolent Crucible will tell you which party member(s) can enter.
These are excellent areas to test your fighting prowess as each of the different playable characters and gain substantial rewards. You can challenge each crucible as many times as you wish, so they are also useful if you need to earn experience.
The final Malevolent Crucible is located on Hexen Isle, so you must complete the main storyline and create game clear save data in order to access all eight. Prove your skills by clearing all eight Crucibles of Malevolence in order to unlock a trophy.