Here's how we determine the name:

* Nickel is the metal cation.

* Sulfur is the nonmetal anion.

* The Roman numeral III indicates that nickel has a +3 charge (Ni³⁺).

* Sulfide is the anion of sulfur, with a -2 charge (S²⁻).

To balance the charges, we need two nickel ions (2 x +3 = +6) and three sulfide ions (3 x -2 = -6).