Deadringer

Summon a 5ft x 5ft x 10ft copper bell that takes a place in initiative order, this bell will stop the caster from making death saving throws. The bell will ring once a turn for the next 3 turns, the first ring will force all spell casters within range to cancel any verbal spell and deploy a barrier around each creature including the caster, this barrier will only reduce damage from magic attacks and weaken all creatures to physical attacks; The second ring will give all enemies within range disadvantage on magic attacks and give all allies advantage on physical attacks. The caster can now choose to make a DC 8 strength saving throw that will stop the final ring, if the final ring is prevented the caster will start regular death saving throws; The final ring will drop all enemy HP to 10 but will kill the caster.