The Keystone Halo is a Demon Unique relic with no boss attached: any drop at Threat 11 or higher can turn out to be it, and at that Threat 0.8% of what falls comes up Unique. It is built on Kindled Keystone Relic — step 4 of the Keystone Relic line, item level 45 and up.

It always rolls the same numbers: +7% Ultimate Generation, +7% Stone Damage, +8% Ultimate Generation, +8% Stone Damage, +24% Ultimate Generation, +130 Maximum Health and +22% Healing Power.

On top of that it carries The Keystone Halo — Ascendant Form heals 18% of max health instead of 10%, over twice the radius, for 2 more allies.

It fires Mason's Eye on its own: Every 5th critical strike cracks the target's guard, building Crumble for 60% of the hit.

It is Adventure Mode loot.

Wearing it hands you Ascendant Form.

“Hold the arch up and the arch holds everyone.”