Source: Icewind Dale - Rime of the Frostmaiden

Wondrous item, legendary (requires attunement by a spellcaster)

A mythallar looks like an enormous crystal ball held in an ornate cradle. The globe sheds bright light in a 300-foot radius and dim light for an additional 300 feet. The globe draws magic from the Weave that can be harnessed for various purposes. For example, Netherese mages used mythallars to keep their cities aloft and empower their magic items. The bigger the mythallar, the more magic it can hold. The largest mythallars are 150 feet in diameter.

The Ythryn mythallar is a relatively small device — a mere 50 feet in diameter. To attune to this mythallar, a creature must finish a short rest within 30 feet of it, meditating on the mythallar. Up to eight creatures can be attuned to it at one time; otherwise, the Ythryn mythallar follows the attunement rules in the Dungeon Master's Guide. If a ninth creature tries to attune to the mythallar, nothing happens.

All creatures attuned to the Ythryn mythallar can sense when the device is being used. A creature attuned to the device can use any of its properties, but only if all other creatures attuned to the device agree to allow it. The Ythryn mythallar's properties are as follows:

  • While you're on the same plane of existence as the Ythryn mythallar, you can use an action to cause it to fly in any direction you choose at a speed of 30 feet. All matter within 500 feet of the device moves with it. The Ythryn mythallar and all structures held aloft by it hover in place when not in motion.
  • As an action, you can cause one magic item you are holding within 30 feet of the Ythryn mythallar to immediately regain all its expended charges or uses. A magic item recharged in this manner can't be recharged by the Ythryn mythallar again until after the item regains expended charges or uses on its own.
  • You can use the Ythryn mythallar to cast the Control Weather spell without requiring any components and without the need for you to be outdoors. This casting of the spell has a 50-mile radius. For the duration of the spell's casting time, you must be within 30 feet of the Ythryn mythallar or the spell fails.

Touching the Mythallar. Any creature that touches the globe of the mythallar must make a DC 22 Constitution saving throw, taking 180 (20d10 + 70) radiant damage on a failed save, or half as much damage on a successful one. Undead have disadvantage on this saving throw. Any object that touches the globe, other than an artifact or the mythallar's cradle, is disintegrated instantly (no save).