Yes, NECROMANCY is a Scrabble word!

Letter values in Scrabble US: N 1, E 1, C 3, R 1, O 1, M 3, A 1, N 1, C 3, Y 4.

Is it a valid word?

NECROMANCY is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the belief in magical spells that harness occult forces or evil spirits to produce unnatural effects in the world
conjuring up the dead, especially for prophesying

Also: black magic, black art.