Blue Crystal Creature

What — A crystal golem: the guardian bound to the cordoned vein of light-blue/cyan crystal deep in the Menagoles Mines, in a lava-layered chamber where the mine's boring machine once had its drill-head melted by something harder than itself (Session 51). A large (~15-ft) construct — the party even joked "if it had a name tag it'd say Golem" — that emerged from the wall when Lash chipped at the crystal, and was killed the following session (Session 52).

Threat — Defeated. A tough, spell-heavy fight, but its slams and impales almost never beat the party's high AC; the real danger was its environmental spellcasting (a rockfall and reverse gravity), not its attacks.

Observed abilities (Session 52)

• Spellcasting with a spell "well." It casts spells of its own; Fenric's spellbreaker strikes burn its spell slots away from lowest to highest, forcing it to spend them. Innately resistant to certain magical effects.
• Legendary actions. Can cast two spells in a legendary action — a single-target jab and an AoE — one of which Silithane counterspelled.
• Ceiling collapse. Channels magic into the roof to drop stalactites and loose rock: a group CON save, damage minus the save (up to ~24). (Thalia took disadvantage — cave-ins are her greatest fear.)
• Reverse gravity. Fills the chamber with reverse gravity for ~20 minutes (persisting past its own death); creatures either drop what they hold and cling to the floor or are flung 30 ft up to slam the ceiling amid the falling debris (31 bludgeoning, reducible if immune to fall damage).
• Immovable form. Immune to any effect that would move it — could not be whipped, dragged, or lifted by its own reverse gravity.

Resistances / immunities / weaknesses — Innate resistance to some magic; immune to forced movement (immovable form). Its crystalline body shatters cleanly when finally cracked ("like a Prince Rupert's drop").

Loot — the crystal core — On death it collapses into shards around a single watermelon-sized, white "epic-grade" crystal core ("crystal heart"). The core is immovable (immune to being moved against its will) and carries gravity-based properties; the GM flags it as craftable into a two-handed hammer, shield, or armour with a gravity / "do-not-move-me" effect. Lash carries it out.

Status — Defeated (Session 52). Silithane landed the killing critical Green-Flame/Booming Blade; the golem shattered, leaving its crystal core and ~20 more minutes of reverse gravity the party waited out. Party still level 7.

SUMMARY: The deep Menagoles blue-crystal vein's guardian — a spellcasting, immovable crystal golem that dropped the ceiling and reversed gravity but whiffed on the party's AC — killed by Silithane in Session 52, leaving a watermelon-sized "epic" gravity-crystal core for Lash to craft.