Marbles of Shioularoum
Wondrous item, very rare (requires attunement by a spellcaster)
These five marbles are about 1 inch in diameter, with swirling black and red patterns that are always moving. When rolled on a surface, they don't follow a normal gravitational pull, but instead roll toward magical auras, and most strongly toward auras caused by necromancy, rune magic, blood magic, or shadowmancy. Each of them can also be used as an arcane focus.
When you cast a necromancy or abjuration spell through one of the marbles, it crumbles to dust, and the spell casts as though you had cast it one level higher (as long as you can cast spells of that level). When you use this ability, all of the marbles whistle like a tea kettle. One of the destroyed marbles reappears beside an intact one every night at midnight, unless a creature is looking at all of them. If all of the marbles have been destroyed before midnight, none of them reappear.