You visit the Silent Realm a handful of times, and not for very long. It's essentially a "dark" version of a small area of Hyrule, around which 18 collectible orbs have been placed. Each orb grants you 90 seconds of immunity from three hulking great guardians - if your immunity expires, they wake up and chase you around.
It's actually quite scary as a single attack from a guardian forces you to start all over again. The trick is to collect the orbs in the correct order, so that you remain immune throughout the whole thing. Is it any good? It stressed me out a bit, to be honest, but was incredibly satisfying to complete.