I interpreted it that the Void Elves maybe start to summon creatures of the Void? And this is the endless stuff?
They just could also indicate that the Void Elves just did something the person who says the quote can't believe. Like for example opening rifts to the Void to summon creatures from it, which would be an unbelievable and horrorfying act, both of power and in a morally sense that they are not above doing something like this.The Void Elves... they just... their numbers are endless...