Tonight, United States audiences finally get to begin watching Merlin season 4.

We’ve been covering this season for several months as its been airing in the United Kingdom since October.

The Darkest Hour Part 1 airs at 10 PM ET/PT on Syfy. Below is a synopsis:

Merlin is to face his toughest challenge yet when Morgana’s blind determination puts at risk Arthur’s future as well as the very balance of the world. With her unbounded powers at their highest, the sorceress uses powerful magic to tear the mystical veil between worlds making the mighty Cailleach to come forth.

Hellish creatures are released and wander around, killing everyone who succumbs to their touch. With Uther but a mere shadow of his former self, it comes down to Arthur and his loyal knights to safeguard the kingdom.

However, swords won’t be enough to vanquish the newest threat and Merlin is utterly shocked to find out what it’ll take to restore the equilibrium of the world: a sacrifice of unimaginable proportions is required.

Watch a video preview below: