Caltrops will remove half the Attack points from a random active Mounted card on the opponent's field during the War phase, before damage is assigned. If there are no applicable cards on the board, the ability will not activate until one is added.
Caltrops, from Old English word 'calcatrippe' and the Latin word 'calcitrapa' meaning "foot trap", are an antipersonnel weapon made up of two or more sharp spikes or spines arranged so that when it is thrown on the ground, one spike always sticks up. The weapon was mainly used for slowing down the advancement of horses, war elephants and armies. They were particularly effective against the soft feet of camels. They are still considered effective in modern times against vehicles with tires.