The Light Repeating Crossbow can be found in Out of the Abyss, page 224.
This magic weapon grants a +1 bonus to attack and damage rolls made with it. While holding it, you can take a Bonus Action to cause it to shed Bright Light in a 30-foot radius and Dim Light for an additional 30 feet. You can extinguish the light as a Bonus Action.
The weapon has 4 charges. You can take a Reaction immediately after being hit by an attack roll to expend 1 of the weapon's charges and force the attacker to make a DC 15 Constitution saving throw. On a failed save, the attacker has the Blinded condition until the end of its next turn. The weapon regains 1d4 expended charges daily at dawn.
This crossbow is fitted with a cartridge that can hold up to six crossbow bolts. It automatically reloads after firing until the cartridge runs out of ammunition. Reloading the cartridge takes an action.
A weapon that can be used to make a ranged attack has a range shown in parentheses after the ammunition or thrown property. The range lists two numbers. The first is the weapon's normal range in feet, and the second indicates the weapon's maximum range. When attacking a target beyond normal range, you have disadvantage on the attack roll. You can't attack a target beyond the weapon's long range.
You can use a weapon that has the ammunition property to make a ranged attack only if you have ammunition to fire from the weapon. Each time you attack with the weapon, you expend one piece of ammunition. Drawing the ammunition from a quiver, case, or other container is part of the attack. Loading a one-handed weapon requires a free hand. At the end of the battle, you can recover half your expended ammunition by taking a minute to search the battlefield.
If you use a weapon that has the ammunition property to make a melee attack, you treat the weapon as an improvised weapon. A sling must be loaded to deal any damage when used in this way.
This weapon requires two hands to use. This property is relevant only when you attack with the weapon, not when you simply hold it.