When a demon experiences extreme rage or sorrow there is a chance, they can embody their ancestral form.
Optional Rule - Animal Instinct: You also attack player creatures that fit into the category of easy prey.
EASY PREY
A creature that is unconscious, restrained, prone, blinded, paralyzed, frightened or bleeding profusely. The DM may consider other circumstances that would define a target as easy prey or not (children, weakly humanoid, disarmed targets, surrendered enemies, etc.)