You see, the Realm of Light Magic is just as strong as the Realm of Dead Magic. And that means that our Realm's power is divided up among all four of the Wizards of Light Magic. On top of that, eons ago, the hordes of the creatures that reside within the Realm of Dead Magic were banished to this place called the abyss. In some ways, it's sort of like a mystical prison that keeps the essence of these creatures trapped. They exist outside both the natural world and the Realm of Dead Magic. They can't free themselves on their own.
Right now, only the Wizard of the Dead Magic has the power to free creatures from the abyss. There are relatively weak creatures there, like goblins and zombies, that have a weak essence. It doesn't take the Wizard of the Dead Magic much effort to summon them. But there are also more powerful creatures, like trolls and ogres, who possess a much stronger essence - and much more power. Those take a whole lot more effort to summon, and that can quickly drain the wizard's spark.
But there are also a select few creatures within the abyss - Lords of the Dead - who have almost as much power as a wizard. They are the ultimate masters of many of their weaker counterparts. I would imagine there is a Lord of the Dead to lead goblins, one to lead zombies, and maybe countless more.
But I haven't had to fight any of them in the past. You see, it takes years - and sometimes even centuries - for the essence of a Lord of the Dead to be ready for summoning. And even then, a Wizard of the Dead Magic has to be strong enough to summon them. But if they are summoned, it can spell disaster for any Wizard of the Light Magic who happens to be around. Thankfully, I've never had to encounter one...