Only a Master Necromancer is fit for the Black Periapt, because other Lords won't waste their points to wield it, having much better options.
Now, yes Black Periapt IS a big deal when you want magic supremacy on the table. Casting spells is still unreliable, but it tips the odds in your favor regarding:
- An Enemy has a magic heavy list. You cast your own spells and then, instead of spending the last 2 Power dice, you save them for the dispel game, where you'll need them most.
- An Enemy has a low magic list. This will wreck the enemy's army, because you can cast your own spells and when you need to dispel, you'll almost always manage to save those 2 dice for later use during your own turn. Cast as many of Remains in Play spells as you can here!
- When you or your opponent manages to get IF on his casting. When that happens, you'll most likely end up with very few power dice of your own, making the casting of further spells useless against your enemy's Dispel Dice pool. So save them, even for later use, since you can carry them over from your dispel dice to your power dice (no need to use them to dispel, if it is not critical). Furthermore, if your opponent casts with IF, you won't throw away all your dispel dice pool and manage to salvage 2 of them.
- During the first turn of the game, most armies don't have that many spells to cast, because there are no ideal targets in range. Vampire Counts is one such army! At most you want to cast Invocation of Nehek on zombies, a bubbled Danse Macabre and maybe Gaze of Nagash against skirmishers, but that's it. Those initial 2 dice can be saved for much later use in the game.
All in all, take the Black Periapt when you have a dual-Master Necromancer list (one with Vampires, one with Death) or if you are playing at high points, to pair him with a Ghoul King or Lv1 Vampire Lord. Alternatively, use it along with Heinrich Kemmler for more awesomeness.
I'm sure there are mor comboes with this arcane item out there, these are my own.