Yes, MANDAMUS is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, N 1, D 2, A 1, M 3, U 1, S 1.

Is it a valid word?

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

an extraordinary writ commanding an official to perform a ministerial act that the law recognizes as an absolute duty and not a matter for the official's discretion; used only when all other judicial remedies fail

Also: writ of mandamus.