Worldswell
Weapon (warhammer), legendary (requires attunement)
Worldswell is an intelligent warhammer, and one of five crafted by dwarves in their war with the elementals. This warhammer was made to fight creatures of earth, and its driving force is their destruction, although it also wishes to protect dwarves at any opportunity.
You gain a +1 bonus to attack and damage rolls made with this magic weapon as well as spell attack rolls made while holding this warhammer. While Worldswell is on your person, you can also read, speak, and understand the dwarven language and gain a +1 to Charisma checks when dealing with dwarves.
In the hands of a dwarf, Worldswell gives a +2 bonus to attack and damage rolls as well as spell attack rolls.
This weapon was crafted by Prime dwarves to fight elementals invading their world. It deals an extra +2d6 damage to creatures of the elemental type, and they must make a DC 15 Constitution save when struck or lose all damage resistances (but not immunities) until the start of your next turn.
While wielding Worldswell, once per day you can expend a Channel Divinity daily usage, replacing any references to a creature type with the elemental (earth) type. If a creature would be automatically destroyed by the outcome of this ability, the elemental is instead controlled you. This control lasts until you release the elemental as a bonus action, the elemental is reduced to 0 hit points, or you and the elemental are on two different planes.
Sentience. Worldswell is a sentient lawful good warhammer with an Intelligence 10, Wisdom 13, and Charisma 14. It has hearing and darkvision out to a range of 60 feet. The weapon communicates empathically through emotions transferred to the wielder.
Personality. Earth elementals are to be killed, no quarter given. If outright combat is suicide, then perhaps someone more worthy of wielding Worldswell will find it next. Dwarves are to be respected and helped at any given opportunity. If the wielder disagrees with any of these things, then Worldswell will force a Charisma check. If it wins, it will make demands to the wielder. If these demands are refused, then the weapon can withhold the powers it confers. Worldswell can, once per day, attempt to take control of its wielder for 1d12 hours. In this case, make a DC 14 Charisma saving throw to resist.
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So, this is for a Planescape game, and the dwarves who crafted this item actually crated other powerful weapons to combat other elementals. A few details about my game are that there is a priest of Kossuth who likes fire and distrusts all other elements (he's a bit of a fanatic), and the game mostly takes place in Sigil where there are lots of creatures that Worldswell is not going to get along with and who the PCs are going to have complicated relationships with because of the general nature of Sigil and the dealings therein. Right now the plan is for the campaign to end up focusing quite a bit on Inner Planar politics, so this warhammer's personality is going to play a fairly interesting role in how dealing with creatures like Dao will play out as well. So yeah, I'm super excited to add this potentially overpowered weapon to my low level game! It's going to be fun to see how the PCs react when it starts to disprove of PC conduct or - potentially - dictate PC conduct!
It also has an interesting relationship with a sideplot of the campaign where the PCs want to find other dwarven survivors of a destroyed Prime Material world. So it serves as a McGuffin, plothook, and has the potential to be removed from the game (given back to its original owners) when that plot line is wrapped up.
And despite being several paragraphs and looking really impressive, 95% of the time it will just be a +1 warhammer that can potentially dominate whoever is holding it.