Anathema (赤の呪い Aka no Noroi, lit. Crimson Curse in the Japanese version, Manteau maléfique in the French version, Maleficio in the Italian version, Maldición in the Spanish version) is a skill from Fire Emblem Awakening that reduces avoid and critical avoid of all enemies within 3 tiles of the user by 10. It is a skill learned by Dark Mages at level 10.
Anathema is a term originally used by religious organization to refer to anyone exiled, banished, or excommunicated from their organization. The root of the word comes from Koine Greek words, ἀνάθεμα (anáthema) meaning "something dedicated especially to evil." Hence why it is learned by Dark Mages.