Nine Men's Morris is one of the oldest board games still played — boards have been found carved into Roman and medieval stonework. Each player has nine pieces, placed and then moved along lines, and forming a mill of three lets you remove one of your opponent's pieces.
You win by reducing your opponent to two pieces or leaving them with no legal move. Both conditions matter: strangling a position is as valid a win as capturing your way to one.
Three of your pieces in a row along a board line. Forming one lets you remove one of your opponent's pieces from the board.
Two mills sharing a piece, so moving that piece back and forth re-forms a mill every turn and removes an opponent's piece each time. It's usually decisive.
Very — boards have been found carved into Roman and medieval stonework, making it one of the oldest board games still played.