You need to have a Link Cable (an in-game item) if it normally requires trade to evolve and a Reaper's Cloth. You also need to Have Recruited Darkrai, Cresselia, Palkia (questionable), and brought back Mananphy (?). Then you should be able to evolve your Pokemon at a special Route.
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you need Reaper Cloth to evolve it. I tried but it somehow doesn't work
Curolundo's answer: Not just reaper cloth alone you also need a link cable.
i think you have to be at least 37, and you need a reaper cloth and a link box. and that is all i know.
First train it to lvl 37 then give i a reaper cloth and link cable and go to the Luminous spring
You have to have a reaper cloth and a link cable, and then go to luminous spring.