The Reaving Dread was a mercenary group, but describing them as such would be quite inadequate. They were not mere mercenaries - they were the best of the best, the most ferocious and capable fighters, able to outperform anyone within their rank.
It was quite common for members of their mercenary group to fight above their level, and with the special augmentations provided by the Condottiere, they were also especially difficult to kill.
It was exactly those features of theirs that allowed them to survive for so long in Abaddon, while everyone else who tried to accompany them died. That was not to say the reinforcements were weak, or didn't help. They were quite strong, and relieved a tremendous amount of pressure off of them. But ultimately, they all fell short.
