Wow, this shield has a rather dramatic side effect.
I'd suggest the following:
When the shield catches arrows in this manner, it stores the damage and slows dissipates it while the shield is held. If taken off before the energies have dissipated, then the bearer takes the damage. Rate of dissipation of course is left to the GM, say 1 hour per arrow.
Perfect simple item. I think these are absolutely rife in Skyrim :)
Mystic Moons idea is probably the best way to use these, but one could add complications to their use. It is possible to create multiple marker stones and some may draw the user to traps or dangerous areas. I would have these always lead to perilous adventure, and be found occasionally in treasure.