Which is the most ferocious army in history?

Assyrians, no questions.

While many would answer “Mongols” because of their brutality and some would answer “Aztecs” or “Soviets” or “Waffen-SS”, the Assyrians were a terrifying and exceptionally brutal army even in the Bronze Age Semitic frame of reference. Even their contemporaries agreed the Assyrians were exceptionally cruel, and the Bible actually gives a polished image of them.

Every religion creates a society of its image, and the Semitic Polytheism created an exceptionally brutal, atrocious and bloodthirsty society. The Bronze Age Semites believed the various gods were in continuous war with each other, and this was reflected on secular issues. Unlike Indo-European Polytheism, Semitic Polytheism was an exceptionally intolerant, fanatic and bloodthirsty religion, with human sacrifices, ritual mutilations, ritual castrations and cult prostitution being the norm. Unlike Zeus, Odin or Juppiter, deities like Baal, Marduk, Ishtar, Nergal and Molech strike horror even today’s psyche. (Nurgle, anyone? Yup, he is Nergal.)

The Assyrian army was based on heavy four horse chariots, light ridden cavalry and heavy infantry (pike men and archers). It was supplemented by levies provided by enslaved and subjugated nations. It was highly professional and disciplined, which made it feared. And exceptionally brutal, which made it horrifying. They pioneered the use of terror as a weapon—and they made the lives of their enemies a living horror story.

The Assyrians created tablets that showed them torturing their enemies to let the next city know what was coming. These showed them skinning their victims alive, blinding them, and impaling them on stakes. See 10 Horrors Of Being Invaded By The Assyrian Army - Listverse

Ashurnasirpal II has left a whole series of these tablets behind, and the descriptions are positively terrifying. “I flayed many right through my land and draped their skins over the walls,” he boasts in one. “I burned their adolescent boys and girls . . . A pillar of heads I erected in front of the city.”

The Chronicles and II Kings contain vivid descriptions of the Assyrians. Kingdom of Israel managed to resist their onslaught for almost three centuries - King Ahab fighting a stalemate with them in the battle of Qarqar 853 BC, but Israel finally fell to Sargon II in 722 BC. The result was complete destruction of Israel and its ten tribes.

The Assyrian kingdom fell after a civil war in 612 BC. The oppressed nations rose in rebellion, and they conquered Nineveh. The whole Empire was destroyed and Nineveh itself was reduced into a heap of rubble. It was forgotten so completely that the Biblical accounts were considered as mere legends. The last Assyrian stronghold fell in 609 BC. The ruins of Nineveh were finally discovered in 1842, and only then the Biblical descriptions were confirmed; yes, the Assyrians were really that brutal.