Nexus Clash

Somewhat similar to SignedName's suggestion but different in flavor.

Child Skill of either Oath of Steel or Bond of the Mechanical (or another skill entirely if appropriate).

For an AP/HP cost the Fallen sheds their exoskeleton (mechanical parts), leaving behind a convincing double. Removal of their mechanical self reduces their potency* and leaves them vulnerable**.

The double appears as an identical character sprite on the map, and as a player character in the description pane. The double last for n minutes or ticks. When interacted with directly*** by another character (healed, damaged, item given etc) the double collapses, inflicting severe debuffs on them for a set duration. The target gets reduced soak and defence. In addition, they get increased cost to movement, and lose HP and MP every time they move to a different tile until the debuff wears off or is removed. Fallen can track the debuffed target until it wears off.

(intent is to keep the target close to you so you can set up bait to lure them in, then spring on them once they're weakened by it)

*disables parent skills for n minutes or ticks
**reduces soak by 2 and defence by 10%, can go into negative
***talking, whispering, using aoe abilities etc should probably not trigger it

Intriguing, but the amount of under-the-hood changes to core systems (which would make those systems way more complicated to do any future work on) we'd need to do to have a double on the map that behaves like a player character way, way outstrips the possible value of the skill.