Glomus

Native to the quasi-elemental plane of Mineral and almost never met anywhere else, a glomus looks like a dense ball of crystals bristling with sharp spires at every angle. A kind of pseudomagnetism locks the crystals into a rough sphere 1d10 feet wide, with 1 Hit Die per foot. It floats calmly through its home plane by levitation, combined with a passwall-like effect that lets it move through any substance.

It attacks by slamming into foes, dealing 1d4 per Hit Die, so a 6-foot, 6 HD glomus hits for 6d4. Its crystalline hide is AC 0, and weapons of less than +2 cannot hurt it at all; a nonmagical weapon that strikes it shatters. Blunt weapons of +2 or better deal double damage.

At 0 hit points the bonds fail and the creature bursts violently, spraying shards that deal 1d4 per Hit Die to everyone within 20 feet, halved by a save vs. breath weapon. An hour later the crystals begin drawing back together, needing one day per Hit Die to rebuild. Only damage beyond twice its total hit points, which pulverizes the crystals, destroys a glomus for good.