Megalodon's Fury

Weapon (greataxe), very rare (requires attunement by a barbarian)

The head of this axe is the sharpened and serrated edge of a massive shark's tooth. You gain +2 bonus to attack and damage rolls made with this magic weapon. The bonus increases to +3 if the target of the attack doesn't have all its hit points.

While the axe is on your person, you gain a swimming speed equal to your walking speed. If you're also underwater, your skin becomes sharp like a shark's: any creature within 5 feet of you that hits you with a melee attack also takes slashing damage equal to your Rage Damage bonus.

The weapon has a number of charges equal to the maximum number of times you can rage before finishing a long rest (a maximum of 6). It regains all expended charges whenever you finish a long rest while the axe is within your reach. You must be raging to expend charges from the axe.

Riptide. When a creature that you can see within 5 feet of you attacks a target other than you, you can use your reaction to expend 1 of the axe's charges to force the attacker to target you instead. Hit or miss, you can then make a melee attack using the axe against the attacker as part of that reaction.

Undertow. When you make an opportunity attack against a creature as a result of it leaving your reach, you can choose to expend 1 of the axe's charges. When you do, you can then follow that creature's movement, up to half your speed, without provoking any opportunity attacks. If the triggering creature continues to move after you stop, you can make another opportunity attack (no action required) against that creature as it leaves your reach for a second time.

"Few earn the sea’s respect—and fewer still survive its fury. This tooth is proof of both: a token of the storm’s wrath, and a weapon forged in its wake."