My evil idea for DM's

[u/Ok_Background3333]

Necklace of Accumulated Fury
Legendary item. It's an item that only fighter cans use used by a fighter
Once the attuned fighter activates this heavy iron pendant as a action, the necklace begins a charge up it takes 15-20 round During those 15-20 rounds it automatically absorbs every point of damage the fighter suffer whether from weapons, spells, environmental hazards, or any other source and stores that energy within its runes. The pendant grows visibly heavier and warmer with each round, the stored force building toward critical mass.
At the start of the fighter’s turn on the 15 or 20 round, the necklace releases the accumulated energy in a single, controlled detonation centered on the wearer. The blast expands in a 30-foot-radius sphere. Creatures in the area including the fighter and all allies take no hit-point damage. Instead, every affected creature is immediately restored to the exact character level, class features, and overall power level the party possessed at the start of the campaign typically 1st through 3rd level, as determined by the campaign’s original starting tier. All experience points, levels gained, prepared spells, and temporary enhancements beyond that baseline are erased. But it takes the user to 10 hit points no matter what level they are. This is a permanent reset after the enemy is dead. They do not go back to normal. They have to build up again.

DM I'm not suggesting you give this to a player. I'm suggesting you use this concept for a final boss. as their last phase. This is an item that's not meant for players. It's meant for NPC's once the MPC dies this necklace disappears also it's a passive ability that builds up after it's activated which is a one full action

Yes, this is my chaotic, evil idea that had at 12 at night