Gomoku
Players alternate placing black and white stones on the intersections of a 15×15 grid. The first to form an unbroken line of exactly five stones — horizontal, vertical, or diagonal — wins. There is no capturing and no moving once a stone is placed, so every move is permanent. Create "open" threes and fours that threaten in two directions at once; a double-four or a four-plus-open-three is usually unstoppable.