The Dr. Who spinoff Torchwood has tried to straddle the line between action and horror, and to my mind, succeeded in being neither.
That stopped on the third season, the five-episode miniseries called Children of Earth. This series is concentrated nightmare fuel.
The aliens come down, and they aren't friendly. Unlike most Dr. Who-style aliens, we don't see what they are. They breathe a poisonous, opaque atmosphere, a very good counterpoint to the "show the monster" in The Host. The aliens are capable of wiping out humanity (shades of colonialism and War of the Worlds), and they can control our children. Children all over the Earth stop what they are doing and start chanting things. It starts with "We are here."
In Episode Four, we get down to brass tacks. A government representative negotiates with the alien, who speak with a breathy, synthesized voice. It's good and creepy. But the script really shines as the humans negotiators come to realize exactly what the aliens want.
In Episode Five, we learn why, and it's worse than we thought. It is one of the most horrible, chilling revelations I've ever seen put on film. The slow negotiation builds up the tension incredibly well. Brilliant and frightening.