[Primordial] Artifact - Primordial Brand

This weapon is the culmination of the four elemental weapons, merged into one. The artifact essentially solves the question of attuning to all four weapons by combining them into one. Of course, this is an Artifact level item, so its extraordinary power, while wieldable, is granted solely at the GM's discretion based on how the story requires they be fused.

The stats include a suggestion of another item which I may or may not illustrate, which are basically powerful elemental crystals. Those crystals carry a portion of the original design ideas for these weapons, which were cut to keep the weapons from being too feature-bloated.

Also I do like how the elements in this weapon intertwine.

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The four elements of this sword twist around and within each other in perfect harmony, a pact forged by the spark within their four component weapons, carefully commanded by the prime essence sliver embedded within. A relic of this immense power has never been seen before, as the four weapons had only been used separately in their campaign against the primordial titans. Yet, each weapon hums a resonant tune longing for unity.

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Primordial Brand
Weapon (shortsword), artifact (requires attunement)

This artifact is the culmination of four elemental weapons: the Arcstratos Shard, the Eminence Spire, the Infernus Halo, and the Torrential Lash. Once combined, typically with the aid of another powerful relic such as a fully merged Sliver of Prime Essence, they become inseparable until their purpose is complete.

Elemental Brand. You have a +3 bonus to attack and damage rolls made with this magic weapon, and you deal an extra 1d10 damage of your choice of bludgeoning, cold, fire, or lightning to targets you hit with it. If the target is an elemental, the extra damage increases to 2d10 and you can brand the elemental with a visible magical mark. You always know the direction to the branded elemental. The brand lasts until you place it on another elemental or until removed by a remove curse spell.

Primordial Mastery. You have resistance to bludgeoning, cold, fire, and lightning damage while you wield this sword. Additionally, you can cast dominate monster from the sword targeting only elementals (spell save DC 20). Once this spell is cast, you can't cast it again until the next dawn.

Sum of Four. While you wield this magic weapon, you can use a bonus action to transform it into the Arcstratos Shard, the Eminence Spire, the Infernus Halo, or the Torrential Lash. The transformation lasts until you use another bonus action to transform it back into the original form, until you transform it into another one of weapons from the list, or until you end your attunement to this weapon. While transformed, you can use it as if it were the chosen weapon and you gain its benefits as if you were attuned to all four weapons. The chosen weapon also improves in the following ways:  

• Arcstratos Shard. When you hit a target with this weapon, one creature of your choice within 5 feet of the target takes 1d6 lightning damage. 
• Eminence Spire. The ground within 15 feet of you is difficult terrain for your enemies. 
• Infernus Halo. Whenever an attacker within 5 feet of you hits you with a melee weapon attack, it takes 1d6 fire damage. 
• Torrential Lash. When you make an attack on your turn with this weapon, your reach for this weapon is 5 feet greater than normal.