Diabolos' battlefield is interactive and it can cause you to die if you are not able and willing to move in the midst of the battle.

If you're wondering what happens exactly, Diabolos causes the floor to fall from under you and you will drop into a pit below. The pit contains extremely difficult monsters to kill and if one party member falls down there alone, they'll be defeated by the monsters; a missing party member or members can contribute to the downfall of the entire party because every party member must contribute to the fight.