Adrans Obsidian Scales.

Once the spell has been casted, a centimetre thick layer of flawless, smooth obsidian scales, like that of a dragon, appears over the targets body. Until the spell ends, the one willing target you can see has a natural AC of 18 (if the armour class over 18 this has no effect), target gains 150 temp hit points. When spell ends or all temp hit points are used up, the obsidian scales on the targets skin shatter like glass, dealing 4d6 piercing damage to all creatures in a 40 ft radius. Piercing damage increase by 1d6, and health by 10, for every level over 5.