I'm playing in a very high level campaign with a load of powerful characters, and I'm trying to get an edge. I homebrewed this magic item and I want you guys to see how powerful it is. For comparison, one of the players (a turncoat) can summon a CR 24 creature once per day.
Sealed Power/Seven Wishes
Weapon (longsword), Legendary (requires attunement)
This weapon is an incomplete longsword which has seven grooves on its hilt. You have a +3 bonus to attack and damage rolls with this weapon. In addition, you can use Dexterity for the attack and damage modifier for this weapon, instead of Strength. This weapon also ignores slashing resistance.
Soul Harvesting: This weapon is incomplete. It requires seven souls to fill the grooves on its hilt. When you reduce a creature to 0 hit points, you can carve out its soul and place it into one of the grooves in the hilt. An addition forms onto the main blade which adds a passive ability to this magic item. Work with your DM to determine the appearance and passive ability of the new addition.
Additional Blades: Each additional blade adds 1d8 additional force damage to your damage rolls.
Blade Ring: If you have at least one additional blade, you can detach them all to form a blade ring. While the blade ring is active, you gain the following features:
Multiple Attacks: For every additional blade you have in the blade ring, you can attack one additional time when you take the Attack action. The benefits from the Additional Blades feature, and the passive abilities from each blade still apply when the Blade ring is active.
Cannon of Light: As an action, you can expend a spell slot to create a beam of light in front of you. The beam manifests in a 120-foot long, 20-foot wide line centered on you. Any creature in the line must make a Dexterity saving throw(DC = 10 + proficiency bonus + amount of additional blades you have), taking 1d10 per level of the expended slot + 1d8 times every additional blade you have as radiant damage on a failed save, and half as much damage on a successful one.
The blade ring lasts for 1 round, unless you expend a spell slot to extend the duration by another round. When the blade ring is dismissed, the blades attach to the main weapon, and your hit point maximum is reduced by an amount equal to 10 times the amount of rounds this ability was active, and it returns to normal after a long rest. If your hit point maximum reaches 0 because of this ability, you instantly die.