Yes, LETHE is a Scrabble word!

Letter values in Scrabble US: L 1, E 1, T 1, H 4, E 1.

Is it a valid word?

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

(Greek mythology) a river in Hades; the souls of the dead had to drink from it, which made them forget all they had done and suffered when they were alive

Also: river lethe.