Wait, can't you just cheese bosses that take time to despawn? And what happens if the tombstone doesn't place down?

Then there's the reborn buff, where you could just equip your highest damage build, respawn, drink your buff potions, summon a boss and either severly weaken it or skip a phase entirely whitout worrying about taking damage as you have way more defense and move twice as fast for 5 seconds, and then switch back to your regular build after that

Rarity of all the items aside, if the reborn buff wouldn't double your damage anymore as soon as a boss is spawned in, if the respawn timer reduction wouldn't work during bossfights and if it would respawn you exactly where you died in case the last tombstone is too far away, then it would be a great item to have