The Grim Gargoyle is a former Earth/Air Epic Boss; it drops Obsidian Fragments, which are used to craft the Super Rare Armor Monstrous Garb. The Grim Gargoyle can currently be found in Heroic Mode stage Relic Ruins and at the Valor stage of Guardian's Crossing.
Recreation of the Grim Gargoyle, might be missing a few parts though