Arm Blade (5e Equipment)
From D&D Wiki
Martial Melee Weapons
The arm blade consists of a short sword with a perpendicular handle attached a third of the way down the length of the sword, and is about 15–20 inches (380–510 mm) long. It is similar to a tonfa, but some of them are fixed to the arm by straps.
Special: since the body of the sword cover your forearm, you can easily use the arm blade defensively. When wielding this weapon, you can use your bonus action to add +1 to your AC until the start of your next turn, if you haven't made an attack with it on your turn.
Usually this weapon is used on pairs, to benefit of this ability.