The Yklwa can be found in Tomb of Annihilation, page 32.
You have a +3 bonus to attack and damage rolls made with this magic weapon. When this weapon reduces a creature to 0 hit points, the creature doesn't die. Instead, infernal runes appear as if carved into their flesh and they're healed to 1 hit point. They now have the paralyzed condition until the condition is removed by a Lesser Restoration spell or similar magic. When the condition is removed, the runes disappear. At the start of each of their turns while they're paralyzed, the creature suffers immense pain and gains a level of exhaustion.
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.