They didn't bother to bring any other type of race for the defense of the realm because they represent the power of the entire realm and its inhabitants, both living and undead.
It is why the realm was named after them, even though they don't have a realm lord to make that decision. Both the living and the dead, gods and demons, fear them. No one else is worthy of the name.
Being Tyrants, they don't usually get along with each other, but they did so for this event. More than a billion of them are surrounding the realm rift. They don't know what's going to come out of it, but they know that whatever it is will be enemies and invaders. So they know to push back everything that comes through the rift.
Each one of them is darkly skinned, so they made the surface of the divine plane look submerged in a black tide.
A Tyrant asked in disbelief, "So we are really being invaded, and this is not a prank?"
