The Hoopak can be found in Dragonlance: Shadow of the Dragon Queen, page 188.
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.
A hoopak is a sturdy stick with a sling at one end and a pointed tip at the other.
When you make a melee attack with this weapon, you ignore its ammunition property. You can use the hoopak as a martial ranged weapon. If you do, it uses the ammunition property, uses sling bullets, and deals 1d4 bludgeoning damage on a hit.
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.
When making an attack with a finesse weapon, you use your choice of your Strength or Dexterity modifier for the attack and damage rolls. You must use the same modifier for both rolls.
This weapon requires two hands to use. This property is relevant only when you attack with the weapon, not when you simply hold it.