Ring of Distance (3.5e Equipment)
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This magic ring always comes in pairs, although you do not necessarily know exactly where the other one is. Whoever wears this ring will know the exact distance to the other ring, but not the any other information about it. This distance can be known even when the other ring becomes destroyed or on another coexistent plane, however if on a different plane that the coordinate system cannot be mapped directly onto (basically, anything that isn't the material/ethereal combination), the distance that is known to the user is a random number instead.