Devil Trigger is one of Dante's Hyper Combos. It is a special technique from Devil May Cry that is possessed by demons and half-demons, which allows them to release their full power.
The Devil Trigger lasts for 10 seconds. While transformed, Dante's strength increases by 15% and he regains red health. If you tag out Dante while he's in Devil Trigger, it stops the timer for his Devil Trigger, and his assist attacks keep the 15% power boost. The time restarts when Dante is tagged back in.
Dante also gains 3 new special attacks while in Devil Trigger: