(으)리라(고) - Intermediate Grammar | Korean FAQ
The grammar form (으)리라(고) is an intermediate level form which gets used commonly in both speech and writing. And although it has a bit of an old style feeling, it's still used today and you should be aware of it.
Fortunately, this form is simpler than it looks, and even has a modern equivalent (sort of) that can help it make more sense.
In this video I summarize how to use (으)리라(고) in just 5 minutes.