The Yklwa can be found in Tomb of Annihilation, page 32.
You have a +1 bonus to attack and damage rolls made with this weapon.
When you roll a 20 with this magic weapon, the target takes an extra 7 Piercing damage.
Note: According to the SRD, it is an extra 2d6 Piercing damage, although this is incorrect.
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.