Custom reaction? Invoke Reckoning

[nimrodx]

Age of Ashes has an NPC with a reaction called the above.

You cannot select this reaction as an ability when creating this npc.

How do you add custom abilities such as this..

Invoke Reckoning [reaction] (divine, enchantment, mental)
Trigger The skeletal Hellknight hits a foe with a Strike.

Effect The skeletal Hellknight causes the creature
struck to endure the pain of an Order of the Nail’s
reckoning—a rite of mortification Hellknights of that
order use to focus their minds. The creature struck
feels as if nails are being driven into their body, and
must succeed at a DC 17 Will save or be stunned 1
from the pain. On a critical failure, the pain manifests
as actual puncture wounds, dealing an additional 2d6
points of piercing damage to the victim.

...or is it not possible?