The Light Repeating Crossbow can be found in Out of the Abyss, page 224.
This crossbow is crafted from blackened wood, and its limbs bear pearl inlays depicting constellations. You ignore the loading property with this crossbow. If you load no ammunition in the weapon, it produces its own, automatically creating one piece of magic ammunition when you make a ranged attack with it. The ammunition created by the weapon vanishes the instant after it hits or misses a target. The crossbow has 3 charges and regains 1d3 expended charges daily at dawn.
The crossbow is decorated with three constellations. As a bonus action, you can tap one of the constellations to invoke it, expending 1 charge and producing one of the following effects:
Balance
The next time you hit a creature with a ranged attack roll using this crossbow before the end of your next turn, you or another creature of your choice within 30 feet of you can regain hit points equal to 1d8 plus your proficiency bonus.
Flames
Until the end of your next turn, when you hit a creature with a ranged attack roll using this crossbow, the attack deals an additional 2d8 fire damage.
Rogue
Until the end of your next turn, you have the invisible condition, and anything you are wearing or carrying is also invisible.
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.