Collar of the Crystal Veil (Patreon)

Wondrous item, very rare (requires attunement by a spellcaster)

This platinum collar surrounds a chained web of ornate diamonds with a series of armor-like plates. While wearing the necklace, a translucent, glittering veil surrounds you: your AC is equal to 12 + your spellcasting ability modifier if you are wearing no armor and using no shield. In addition, you gain a +1 bonus to saving throws.

Crystalline Barrier. As a bonus action while wearing the necklace, you can project its magic outward to form a magical barrier. The barrier uses your AC, has 50 hit points, and is immune to poison and psychic damage. It extends from you to form a 10-foot-radius sphere. Nothing can physically pass through the barrier, and spells cast from outside the barrier can't target anything within its area. Similarly, the area within the barrier is excluded from the areas affected by such spells.

The barrier remains for as long as you concentrate (as if concentrating on a spell), to a maximum of 1 minute, or until it drops to 0 hit points. You can designate up to 4 creatures that can pass through the barrier when it appears. The sphere moves with you and remains centered on you. It ends early if you remove the necklace or if you move in such a way that a creature you didn't designate would be forced to enter its area. Once this property of the necklace has been used, it can't be used again until the next dawn.

The ancient dragon gazed down at the sacrificial altar of his followers. Despite age-clouded eyes, it was easy to see the ill-fit robes, the nervous stance, the trembling voice of the one presenting the motley group's offering—nothing but a frail man in a travel-stained cloak—just before he cut them off.

"Enough! Believe you that simple tricks be enough to fool a grand wyrm? Yet I thank you, ill-disguised adventurers; you seem not to know your act has made of you simple fuel for my pyres!"

And he inhaled. The flame inside sparked beyond white-hot before exploding outward, bathing his ritual chamber in a searing inferno.

As the fiery breath died away, lingering tongues of flame revealed crystalline cracks in the air around the adventurers. A network of arcane flecks spiraled outward from the man to encircle his allies and protect them from the blast. At last, the barrier collapsed beneath fading embers, and the heat-filled air flooded into the briefly-sheltered space—blasting the cloak aside to reveal a mantle of crystalline brilliance. The revealed mage smirked.

"We know. And now you've wasted your pilot light."