Crashing Tide Hammer (Patreon)

Weapon (maul or warhammer), rare (requires attunement)

This item takes the form of a metal rod with a slightly weighted, porous end. While grasping the rod, you can use a bonus action to cause a hammer head of swirling salt water to gush forth from the rod and spring into existence. You can use another bonus action while holding it to make the hammer's head disappear. While the hammer's head is summoned, it gains the following properties.

You gain a +1 bonus to attack and damage rolls made with this magic weapon, which extinguishes any nonmagical flame that it hits. In addition, when you hit a target primarily made of fire with the weapon, such as a fire elemental or magma mephit, you treat the weapon's damage dice as having rolled their maximum value.

The first time on each of your turns when you have advantage on an attack roll against a creature and hit it with the hammer, that creature must succeed on a DC 13 Strength saving throw or be knocked prone. Whenever you hit a prone creature with the hammer, you can choose to push it up to 10 feet away from you, sweeping it away in a roiling tide of salt water.

The roar of heated battle. The crash of steel on steel.

The roar of churning brine. The crash of falling waves.

The tide is always changing.