Aeon Disk

Wondrous item, very rare (requires attunement) When you take damage from a hostile creature, if your hit points would go below half your hit point maximum (rounded down), you can use your reaction to make the Aeon Disk trigger and you cast Otiluke's Resilient Sphere targeted on yourself. Once the Aeon Disk has been used, it cannot be used again until the next dawn. ''A powerful artifact long ago smuggled out of the Ivory Incubarium. Or so many believe.''