Certain enemies have small notches at the bottom section of their health bar, indicating that they are benefitting from the Boss Ward system.
When enough damage is dealt to these enemies and their health bar reaches a notch, it is consumed and the enemy is granted ward based on their maximum health. When a notch is consumed that grants the boss ward, they will also be stunned for a short duration.
Unlike the player ward which decays faster the more ward you have, the rate at which the Boss Ward decays speeds up over time.
While only Bosses are able to benefit from the Boss Ward mechanic, not all Boss enemies have it.