"Unrelenting hordes" refers to a large and unstoppable group of people or things that continue to attack or move forward with great intensity and persistence. Full definition
The world as we know it is gone, overrun by unrelenting hordes of infecteds. (pixeljudge.com)
Call of Duty World at War was also the first game to feature the extremely popular Zombies mini-game which is a four player cooperative game where player try to survive as long as possible against and unrelenting horde of Nazi zombies. (lifewire.com)
His demonic invasion of the west is gaining momentum — an unrelenting horde unhindered by food or sleep. (lauramhughes.com)