Source: The Book of Many Things

Wondrous Item, varies (requires attunement by a Cleric or Paladin)

The backs of these cards are inscribed with glyphs representing the Inner Planes, the Outer Planes, or the holy symbols of various deities. While holding this deck, you can use it as a spellcasting focus, and you gain a bonus to spell attack rolls and to your spell save DC. The bonus is determined by the deck’s rarity.

In addition, while you’re holding the deck, you can draw a card as an action to expend and roll one of your Hit Dice and add the deck’s bonus to the number rolled. One creature you can see within 30 feet of you either takes radiant damage or regains hit points (your choice) equal to the total.

Rarity Bonus
Rare +1
Very Rare +2
Legendary +3