Among other things, after conquering a city, they were known for:
1. "Child fires"...burning all the children of that city alive in a mass inferno while forcing the parents to watch.
2. What few children were spared would be sold as sex slaves.
3. Skinning their victims alive and hanging the skins as decorations outside the city walls of Ninevah.
4. Head pyramids: mass beheadings of a conquered people and piling their heads in large pyramids around the city.
5. Adorning the city with strings of severed arms and legs of their victims.
Oddly enough, the most feared aspect of the Assyrians wasn't any of this; it was their practice of forcing a people out of their own homeland and occupying their cities with themselves. Robbing a population of its cultural heritage was considered more barbaric than anything else.
The Israelites suffered all of the above as a consequence of years of idolatry.
Judah *almost* suffered the same fate--but because of the prayers and fastings of repentance led by Hezekiah and Isaiah, Jerusalem was spared at the last minute. This is verified by the Taylor Prism, an ancient clay tablet where the Assyrians bragged about their great military accomplishments. Sennacherib boasts mightily of his huge victories, naming Hezekiah and Jerusalem; then the prism suddenly stops and says no more.
Paul wrote to the Corinthians: I Corinthians 10:11 Now all these things happened unto them for ensembles: and they are written for our admonition, upon whom the ends of the world (age) come...
We have the script of what will be again.