D20:Ring of Friend Shield Magic Ring

Friend Shield: These curious rings always come in pairs. A friend shield ring without its mate is useless. Either wearer of one of a pair of the rings can, at any time, command his or her ring to cast a shield other spell with the wearer of the mated ring as the recipient. This effect has no range limitation.
Moderate abjuration; CL 10th; Forge Ring, shield other; Price 50,000 gp (for a pair).

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System Reference Document -> Magic Items: Rings