If player A controls Legendary Atlantean Tridon and player B has no monsters on his field or all his monsters are face-down, player A can activate Legendary Atlantean Tridon's effect.
That is because the ATK reduction part of Tridon's effect is mandatory and will resolve anyway, regardless of whether or not there are any face-up monsters on player B's field.
That is because the ATK reduction part of Tridon's effect is mandatory and will resolve anyway, regardless of whether or not there are any face-up monsters on player B's field.