Same idea with Runes and other ability Markers. Their owner retains control of them even when you steal the object they're attached to. I'm 95% sure this is the correct answer now that I've thought about it some.
Hmmm. I am still skeptical.
If I read the text in the rulebook: https://gyazo.com/711f05ecc2242aa6c8565c5797e1de18
It seems more like runes work like enchantments which modify the object.
So the runes add a trait/ability to the equipment and because you control the equipment you can use its abilities. For example if the Rune of Fortification is on a breastplate, if you steal the armor, you benefit from the rune and suddenly the armor has 3 armor.
Same with the rune of power, now your equipment has the ability that you can pay 1 less mana for certain actions.
Also true with piercing +1, I doubt that the weapon loses its piercing +1 trait if you steal it. I mean, the rune is forged into the equipment, you can't just decide that "it creases to work" once it is stolen - the equipment is already forged, too late for that :p
Though another interesting question.. do you have to pay 2 more mana if you, as the AT Warlord, want to destroy the equipment?
TL;DR Controlling an object does not mean you gain control of all attached effects
Depends on the rune, and they are:
Rune of Fortification: If this equipment gives an Armor +X bonus to the Mage, it gives an additional Armor +1.
Rune of Power: Once per round, you may pay 1 less mana when casting a spell bound to this equipment or using a spell action provided by this equipment.
Rune of Precision: This equipment’s non-spell attacks gain the Piercing +1 trait.
Rune of Reforging: This equipment gains the Cantrip trait.
Rune of Shielding: If this equipment gives your Mage a Defense, the first time each round that defense is used, add +2 to the Defense roll.
When the FM steals the equipment the rune is still on the item and the "rune" never passes to the FM control.
So the piercing rune for example still gives the pierce +1 because that is not an optional ability and it is always in effect.
The rune of power gives the controller of the rune, not the equipment, the ability to spend one less mana. you here means the AT warlord.
Its the exact same thing as mind controlling a BM pet, it still gets melee ++ for attacking in the same zone as the BM, not melee ++ for being in the FM zone, regardless of who controls the creature.