The Nagelring is a legendary ring in Diablo III. It requires character level 38 to drop. As of patch 2.2.0, it became a Season Three exclusive item, but drops for all characters after that Season ended.
It gained a unique affix in 2.2.0 that summons Fallen Lunatics, to a limit of 4 at a time (8 in early PTR builds). Each of them explodes for roughly 10000% damage in 15-20 yards, choosing the highest Elemental Damage skill bonus the character has. They follow the character as pets. In all other means, they are identical to normal monsters, except for color (to distinguish them easily).
Stats (Level 38)
When this ring first gained a unique affix, it could drop as early as level 8, potentially allowing most foes (including bosses) to be killed by four Lunatics alone, without player lifting a finger.