Yes, SPLENIUS is a Scrabble word!

Letter values in Scrabble US: S 1, P 3, L 1, E 1, N 1, I 1, U 1, S 1.

Is it a valid word?

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

either of two flat muscles that extend from the upper vertebrae to the base of the skull and serve to rotate or flex or extend the head and neck

Also: splenius muscle.