Amulet of Arcane Dominance

Wondrous Item, rare (requires attunement by a Spellcaster)

An amulet set with an arcane crystal that swirls with violet light. To some, it feels like power. To others—it feels like inevitability.

Arcane Instability

While attuned to this amulet, your magic carries a destabilizing resonance.

Whenever a creature fails a saving throw against one of your spells, or you hit a creature with a spell attack, the target gains 1 stack of Arcane Instability.

Spells of 1st level or higher apply a number of stacks equal to the spell’s level.
Cantrips apply 1 stack.
A creature can have a maximum of 5 stacks at a time.
All stacks fade harmlessly after 1 minute if the threshold is not reached.

Arcane Overload

When a creature reaches 5 stacks of Arcane Instability, the instability erupts. Choose one of the following effects:

• Weakened Resistance:
The creature has disadvantage on its saving throw against the next spell that requires one.

• Force Cascade:
The creature takes force damage equal to three times your spellcasting ability modifier.

The stacks are then removed from that creature.

Stability Limit

If a single spell of 5th level or higher would apply 5 or more stacks at once, it cannot trigger Arcane Overload by itself.
The creature must already have at least 1 stack before that spell is cast for the overload to trigger, or an additional spell must be used to trigger the five stacks applied by that 5th level or higher spell.