Bahamut's Divine Sanctuary

Bahamut's Divine Sanctuary is a pinnacle of celestial protection, a manifestation of the Platinum Dragon's unwavering shield against the malevolent forces of Tiamat and her Chromatic Dragons. When this spell is cast, a radiant, almond-shaped magical shield envelops a location of the caster's choice within 120 feet and extends to cover anyone within a 25-foot radius. The shield radiates a brilliant, iridescent glow, signifying the divine safeguard it provides against the destructive powers of Tiamat. The spell was created by Bahamut to give his forces a critical defensive advantage in times of need against Tiamat and her followers.

A scale of a platinum dragon is required to cast the spell, with Bahamut being the only known platinum dragon. You must have the scale on your person, not in a demiplane like in a bag of holding. The user can cast the shield around a single target of their choosing within range (you can choose yourself), the scale teleports instantly in front of the target, facing the source of damage. The 25 foot radius shield forms as a dome of platinum magic instantly, blocking the attack. The shield acts as an impenetrable barrier specifically designed to ward off attacks from Tiamat and her Chromatic Dragons. It is a 9th-level spell and grants absolute protection against any form of damage inflicted by any chromatic dragon or any form of Tiamat's breath attacks or their physical assaults. Furthermore, it denies entry to Tiamat and her minions, acting as an impenetrable boundary that even the Queen of Chromatic Dragons cannot breach without some effort. If Tiamat herself attacks the shield it will eventually fail as she will eventually overpower the small portion of Bahamut's power within the scale that fuels the shield. 

Cover. The creature has total cover.

Damage Immunity. The creature has immunity to acid, cold, fire, lightning, and poison damage.