Wave
Weapon (trident), legendary (requires attunement by a creature that worships a god of the sea)
Held in the dungeon of White Plume Mountain, this trident is an exquisite weapon engraved with images of waves, shells, and sea creatures. Although you must worship a god of the sea to attune to this weapon, Wave happily accepts new converts.
You gain a +3 bonus to attack and damage rolls made with this magic weapon. If you score a critical hit with it, the target takes extra necrotic damage equal to half its hit point maximum.
The weapon also functions as a trident of fish command and a weapon of warning. It can confer the benefit of a cap of water breathing while you hold it, and you can use it as a cube of force by choosing the effect, instead of pressing cube sides to select it.
Trident of Fish Command Effect: This effect has 3 charges. While you carry it, you can use an action and expend 1 charge to cast dominate beast (save DC 15) from it on a beast that has an innate swimming speed. The trident regains 1d3 expended charges daily at dawn.
Weapon of Warning Effect: This item warns you of danger. While the weapon is on your person, you have advantage on initiative rolls. In addition, you and any of your companions within 30 feet of you can't be surprised, except when incapacitated by something other than nonmagical sleep. The weapon magically awakens you and your companions within range if any of you are sleeping naturally when combat begins.
Cap of Water Breathing Effect: While holding this item underwater, you can speak its command word as an action to create a bubble of air around your head. It allows you to breathe normally underwater. This bubble stays with you until you speak the command word again, the item is dropped, or you are no longer underwater.
Cube of Force Effect: This effect starts with 36 charges, and it regains 1d20 expended charges daily at dawn.
You can use an action to expend a number of charges based on the chosen effect, as listed below. If the Wave has insufficient charges remaining, nothing happens. Otherwise, a barrier of invisible force springs into existence, forming a cube 15 feet on a side. The barrier is centered on you, moves with you, and lasts for 1 minute, until you use an action to end it, or the trident runs out of charges.
You can change the barrier's effect by expending the requisite number of charges, resetting the duration. If your movement causes the barrier to come into contact with a solid object that can't pass through the cube, you can't move any closer to that object as long as the barrier remains.
Charges ... Effect
1 ... Gases, wind, and fog can't pass through the barrier.
2 ... Nonliving matter can't pass through the barrier. Walls, floors, and ceilings can pass through at your discretion.
3 ... Living matter can't pass through the barrier.
4 ... Spell effects can't pass through the barrier.
5 ... Nothing can pass through the barrier. Walls, floors, and ceilings can pass through at your discretion.
Wave loses charges when its barrier is targeted by certain spells or comes into contact with certain spell or magic item effects.
Disintegrate spell costs 1d12 charges.
Horn of Blasting item costs 1d10 charges.
Passwall spell costs 1d6 charges.
Prismatic Spray spell costs 1d20 charges.
Wall of Fire spell costs 1d4 charges.
Sentience. Wave is a sentient weapon of neutral alignment, with an Intelligence of 14, a Wisdom of 10, and a Charisma of 18. It has hearing and darkvision out to a range of 120 feet.
The weapon communicates telepathically with its wielder and can speak, read, and understand Aquan. It can also speak with aquatic animals as if using a speak with animals spell, using telepathy to involve its wielder in the conversation.
Personality. When it grows restless, Wave has a habit of humming tunes that vary from sea chanteys to sacred hymns of the sea gods.
Wave zealously desires to convert mortals to the worship of one or more sea gods, or else to consign the faithless to death. Conflict arises if the wielder fails to further the weapon's objectives in the world.
The trident has a nostalgic attachment to the place where it was forged, a desolate island called Thunderforge. A sea god imprisoned a family of storm giants there, and the giants forged Wave in an act of devotion to - or rebellion against - that god.
Wave harbors a secret doubt about its own nature and purpose. For all its devotion to the sea gods, Wave fears that it was intended to bring about a particular sea god's demise. This destiny is something Wave might not be able to avert.
For more info, see Dungeon Master's Guide, p. 218.