This is a quartz crystal which has been attuned to a number of leylines connected to graveyards, memorials, roadside shrines, and similiar locations. By enchanting it to contain a portal, the necromancer can project into the crystal and then travel through the leylines to the desired location. Or they can simply project their mind through the crystal to the desired location in order to watch over it. In addtion it serves as a powerful arcane battery and location focus.
This stone is called a Gravegate.
A man visits a local witch for help and they talk in her sitting room
Man: Ive heard you have are a great necromancer and understand the inner workings of the soul very well.
Witch: Oh, don’t flatter me. I’m mediocre.
Skeleton walking out of the kitchen with a tray of cookies and an apron that says “Suck my marrow”: *hands both a cookie* Glad to know I was raised from the dead by a mediocre necromancer Gladys. Really reassuring that I won’t simply cease to have a physical form at any moment.