Talaria

Mercury's Boots are Mercury Black's weapons of choice.

Mercury is briefly seen fighting in the Volume 2 Opening, and subsequently in "Extracurricular," where he employs an acrobatic fighting style emphasizing kicks in combination with his greaves. He is later seen using them to fend off the Grimm invasion of Vale during "Breach."

Description
Mercury's weapon appears to be a large pair of greaves with a firing mechanism built into them, similar to Yang Xiao Long's Ember Celica. Unlike Ember Celica, however, it appears his boots only fire their ammunition upon impact with a target and cannot be used as ranged projectiles.

Trivia

 * The greaves are likely a reference to the winged sandals characteristic of the Roman god Mercury.