Phorusrhacos, was the second largest terror bird all time (the greatest being Kelenken, which has never been seen in any of the Walking With series, and was about a metre taller and longer)
Even so, Phorusrhacos was the archetypal "Terror Bird" of the Quaternary Period, hunting it's prey for millions of years. Phorusrachos lived from 5 Ma - 10,000/6,000 years ago. However, the line of South American Terror Birds to which it belonged, the Neognath Phorusrhacids, had dominated South America for over 27 million years overall (with the Kelenken for instance, living 15 Ma).
It was an imposing predator, 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 avian heart, like an Ostrich or even Dinosaur) In fact, Phorusrachos would have looked for all the world like a Theropod Dinosaur covered in smooth feathers (or an already feathery saurian Dinosaur)
It would have had glaring 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 ecological balance of power.
Although Walking With Beasts portrays the Smilodon as the clear 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 food. 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 possibly more than a match for most Smilodon. They 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 hunted the ancient horses, and became known to some experts as a new species, called Titanis.