The Light Repeating Crossbow can be found in Out of the Abyss, page 224.
You gain a +3 bonus to attack rolls and damage rolls made with this magic weapon. When you hit a Fiend or an Undead with it, that creature takes an extra 2d10 Radiant damage.
While you hold the drawn weapon, it creates a 10-foot Emanation [Area of Effect] originating from you. You and all creatures Friendly [Attitude] to you in the Emanation [Area of Effect] have Advantage on saving throws against spells and other magical effects. If you have 17 or more levels in the Paladin class, the size of the Emanation [Area of Effect] increases to 30 feet.
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.