My King can't be this Green

After the events of Fate/tiger colosseum UPPER Saber decides to leave and to have some peace along the way ends up helping the owner of an antique shop that gives her the gift of a mask as a form of gratitude. But it's time to go home and entering a new fight against Gilgamesh, the mask is placed on Saber by accident, Could Shirou and and his friends survive this strange night?