A shadow is always near but can never be caught, it can take immeasurable damage but yet not be affected, a shadow is a perfect mimic of the one who casts it.
On the of Myst(ical) end I imagine the ability to call upon or conjure up powers beyond the world of mortals. Incredible strength, forthsight, regeneration.
Curses or hexes as as Stormclock mentioned that can slow or stop opponents.
Ability to absorb energy and weaken or return it twofold.
Teleportation - but maybe that is getting a little beyond gladiatorial context.