Every piece slides like a rook, any distance orthogonally, blocked by other pieces. An enemy piece is captured the instant it becomes sandwiched between two of your pieces in a straight line.
This combines the ancient Greek game of Petteia with its Roman descendant Latrunculi (ludus latrunculorum, "the game of little soldiers"). In both, every piece slides like a rook along straight orthogonal lines, and the only way to capture is by sandwiching an enemy piece between two of your own pieces in a line. Petteia is pure annihilation between soldiers; Latrunculi adds a "Dux" (commander) whose immobilization or capture ends the game.
Simplification: the additional 21st-century tactics from "Latrunculi XXI" (push-and-crush, flank, and phalanx attacks) and corner-based captures are not reproduced here — this is the classical custodial-capture-only ruleset.