Mabaran Obsidian Crystal of Shielding

Arcane Focus: An arcane focus is a special item designed to channel the power of arcane spells. A sorcerer, warlock, or wizard can use such an item as a spellcasting focus.

An orb of shielding is a polished, spherical chunk of crystal or stone aligned to one of the planes of existence. If you're a spellcaster, you can use this orb as a spellcasting focus. If you're holding the orb when you take necrotic damage, you can use your reaction to reduce the damage by 1d4 (to a minimum of 0).