I'm a huge fan of magical reagents: rare gems, extra planar materials, monster parts, etc. The exact reagent required would depend on what items you are trying to convert. This way, you can decide how easy or hard it is to get the reagent.For example, if someone wants to combine a +1 weapon with a vicious weapon to make a +1 vicious weapon, you could decide that this requires 250 gp worth of behir bile. If you want to go easy on the PCs, there's some for sale in town. If you want things to be harder, then there is a shortage on behir bile right now -- it'll cost double, and arrive in two weeks. If you're starting to think that +1 vicious weapons are overpowered, then oops, everyone is fresh out of bile and if you want some you'll have to go kill a behir yourself.
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