What's the difference between...
Gandalf will have killed hundreds of dragons in his time.
Gandalf must have killed hundreds of dragons in his time.
The first sentence is a line from a movie script, and the second one is a something I made up. I've seen a few grammar webpages that says "Will + Have + PP" can be used for past certainty when they are pretty sure about it. (Not future perfect). But how much is it different from "Must + Have + PP"?