You must revive its fossil.
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Archen evolves into Archopes at level 37 in Pokemon Black, hope this helps :)
Archen evolves into Archeops at Lv. 37.
There is a lady with a backpack on she gives you either a plume fossil(Archen) or a cover fossil(tirtouga)
Archen evolves at level 37.
In the first bit of relic castle talk to the woman she gives you the choice of plume(archen) or cover(tirtouga) fossil