Hammer of Thunderbolts 5e

Weapon (maul), legendary (requires attunement)

You gain a +1 bonus to attack and damage rolls made with this magic weapon.

Giant's Bane (Requires Attunement). You must be wearing a Belt of Giant Strength (any variety) and Gauntlets of Ogre Power to attune to this weapon. The attunement ends if you take off either of those items. While you are attuned to this weapon and holding it, your Strength score increases by 4 and can exceed 20, but not 30. When you roll a 20 on an attack roll made with this weapon against a giant, the giant must succeed on a DC 17 Constitution saving throw or die.

The hammer also has 5 charges. While attuned to it, you can expend 1 charge and make a ranged weapon attack with the hammer, hurling it as if it had the thrown property with a normal range of 20 feet and a long range of 60 feet. If the attack hits, the hammer unleashes a thunderclap audible out to 300 feet. The target and every creature within 30 feet of it must succeed on a DC 17 Constitution saving throw or be stunned until the end of your next turn. The hammer regains 1d4 + 1 expended charges daily at dawn.

Proficiency with a maul allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Commentary by Sam West, Twitter: @CrierKobold

Some people really hate giants- if you want to ruin a giant’s day, get a belt, gauntlets, and this hammer to absolutely thrash them.

You don’t need to attune to the Hammer of Thunderbolts to benefit from its “masterwork” +1 quality; if you want a +1 Maul, this will do the trick, regardless as to whether you can attune to it or not.
Attuning to it requires two additional worn items: Gauntlets of Ogre Power and a Belt of Giant Strength of any variety.
The attunement specifies it ends if you take off either of those items; whether or not.
The Hammer of Thunderbolts doesn’t immediately return to you when you throw it; you have to go chase after it and pick it up.
Rolling a 20 isn’t the same as getting a critical hit; a Champion fighter won’t get the giant execute ability when it critical hits with a 19, for example.
As clarified by Jeremy Crawford, your current set Strength score (such as one set by a Belt of Giant Strength) is augmented by the Hammer to a max of 29.

For example, while attuned to the Hammer of Thunderbolts while attuned to a belt of Cloud Giant Strength, you’d add the +4 to the Strength score set by the belt (27) to a maximum of 29, resulting in a Strength score of 29.

Gauntlets of Ogre Power also set your Strength to 19- that’s almost always going to be lower than the Giant Belt options.

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