Cornerstone is a Nightmare Unique ring, hunted from The Mountain Sire in the Mortal Arena, always at item level 120. It is one of four ordinary drops on its list — 22.5% of the Uniques that boss gives up, about 1 in 4 — and every kill rolls the list 5 times. It is built on Immaculate Gravestone Band — step 6 of the Gravestone Band line, item level 80 and up.

It always rolls the same numbers: +52 Armor, +14% Stone Damage, +63 Maximum Health, +38% Stone Damage, +346 Maximum Health and +11% Power.

One Unique drop in 40 is a Mythical copy: the same item carrying 15–25% Mythical Energy that boosts every stat on it and can never be changed; the 15% floor is 10 times as common as the perfect 25%.

On top of that it carries Cornerstone — You have 40% more Armor.

“Everything since was built on this.”