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Douglas Harper's Etymology Dictionary
necro-

before vowels, necr-, word-forming element meaning "death, corpse, dead tissue," from comb. form of Greek nekros "dead body, corpse, dead person," from PIE *nek- (1) "death, natural death" (cognates: Sanskrit nasyati "disappears, perishes," Avestan nasyeiti "disappears," nasu- "corpse," Old Persian vi-nathayatiy "he injures;" Latin nex, genitive necis "violent death, murder" (as opposed to mors), nocere "to harm, hurt," noxius "harmful;" Greek nekus "dead" (adj.), nekros "dead body, corpse;" Old Irish ec, Breton ankou, Welsh angeu "death").

Wiktionary
necro-

pre. Forming compound words related to death or dead tissue.