It was an imposing killer, with an eagle-like hooked beak, for tearing into mammalian prey. They had sharp talons on their thick scaled feet, and their legs were very slim and elegantly tall. They would have been able to run at incredible speeds, faster even than a horse at the races at full pelt (due to the immense stride lengths and efficient bird heart, like an Ostritch or even Dinosaur) In fact, Phorusrachos would have looked for all the world like a Theropod Dinosaur covered in smooth feathers. It would have had glaring, scary eyes, like any bird of prey has similarly. It was the top predator of South America for millions of years, unchallenged by any other predator.
However, as the land bridge formed between North and South America, Sabre-Toothed Cats such as Smilodon invaded, shifting the balance of power. Although Walking With Beasts portrays the Smilodon as the sole masters, the only top predators, even the companion book shows the Terror Birds in a more positive light. The fact of the matter is, that these were 3 metre tall killers, agile and in need of regular kills. They were not pushovers, even for a Sabre-Tooth. What is more, they could have hunted in packs or at least pairs, and one to one the biggest Phorusrachos were more than a match for any Smilodon. The simple truth is, these became joint shareholders of the top predator niche - and would likely have just tried to avoid each other whenever and wherever possible. The deadly Terror Birds, to prove this claim, we know to have moved North into the plains of Texas and Florida - where they terrorised the ancient horses, and became known to some experts as a new species, called Titanis (this is not now accepted in the best circles of Palaeontology, as it seems it was still just Phorusrachos).