Truth be told, Darius had greatly underestimated the amount of undead in the Plains of Death. Describing them as infinite did not seem to be too much of an exaggeration.
He could only bitterly laugh at his former naivety in thinking that hiring those Andrato fools could thin down their numbers. Even if the entire Fallon continent's armies and legions had answered his call, he estimated that they would have only been able to cull, at best, 70% of the undead in these two early bands.
That's not even counting their own troops who would die and later be raised as servants to the Plains of Death.
From the way Thanatos had spoken about them, it was clear that he encouraged invasions to his two early bands. When else were his weaker undead supposed to gain combat experience and rise to become Adept stage experts?
How else would his cannon fodder be able to increase in size without a regular infusion of idiotic living armies who die in battle?