I might even suggest double xp during the entire cooldown period so that the owning guild might get as many as ten OP mats (I think that's the usual rate x 2). If this scheme actually makes OP ownership evanescent, then there does need to be an incentive to spend the money to attack for something you will very likely only hold for a short time.
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Remembering Tyneetryk
Phaedreas Tears - 15 years old and first(*) of true neutral guilds in Atys.
(*) This statement is contested, but we are certainly the longest lasting.
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