A_CentaurDefend

Calls A_FaceTarget. If the target is within melee range of the calling actor, there is a 12.5% chance that the calling actor clears its REFLECTIVE and INVULNERABLE flags and goes into its Melee state.

ACTOR MirrorImp : DoomImp
{
  States
  {
  Pain:
    TROO H 6 A_Pain
    TROO H 6 A_SetReflectiveInvulnerable
    TROO FFF 15 A_CentaurDefend
    TROO F 1 A_UnsetReflectiveInvulnerable
    Goto See
  Melee:
    TROO EF 4 A_FaceTarget
    TROO G 3 A_CustomMeleeAttack(50, "skeleton/melee", "none")
    Goto See
  }
}

This is a modified Imp using the same guard stance as the centaur when it enters the pain state. When the imp is hurt, it will stop moving and become invulnerable and reflective for a short time. But if the player is too close to it, A_CentaurDefend has a chance to make the imp switch to its melee state and deliver a dangerously powerful blow to the player.

A_CentaurDefend works together with A_SetReflectiveInvulnerable and A_UnsetReflectiveInvulnerable, and would not make much sense without the other two in the correct order.