The Yklwa can be found in Tomb of Annihilation, page 32.
This weapon is decorated with dragon heads, claws, wings, scales, or Draconic letters. When it steeps in a dragon's hoard, it absorbs the energy of the dragon's breath weapon and deals damage of that type with its special properties.
Whenever you roll a 20 on your attack roll with this weapon, each creature of your choice within 5 feet of the target takes 5 damage of the type dealt by the dragon's breath weapon.
You gain a +3 bonus to attack and damage rolls made using the weapon. On a hit, the weapon deals an extra 3d6 damage of the type dealt by the dragon's breath weapon.
As an action, you can unleash a 60-foot cone of destructive energy from the weapon. Each creature in that area must make a DC 18 Dexterity saving throw, taking 12d6 damage of the type dealt by the dragon's breath weapon on a failed save, or half as much damage on a successful one. Once this action is used, it can't be used again until the next dawn.
A yklwa (pronounced YICK-ul-wah) is a simple melee weapon that is the traditional weapon of Chultan warriors. A yklwa consists of a 3-foot wooden shaft with a steel or stone blade up to 18 inches long. Although it has the thrown weapon property, the yklwa is not well balanced for throwing.
If a weapon has the thrown property, you can throw the weapon to make a ranged attack. If the weapon is a melee weapon, you use the same ability modifier for that attack roll and damage roll that you would use for a melee attack with the weapon. For example, if you throw a handaxe, you use your Strength, but if you throw a dagger, you can use either your Strength or your Dexterity, since the dagger has the finesse property.