"They're not of Asgard or any world known. He means to lead them against your people. They will win him the Earth. "
Loki (as well as, secretly, Thanos)
They're a powerful, technologically advanced alien race that lead a war against mankind, destroying half of New York in the process.
Of course, it helps that they also have massive flying alien snake things causing a lot of the damage.
Terrifying Or Terrible:
As soon as you get an up-close glimpse of their face, it's easy to see that they're terrifying.
They also very nearly defeat the Avengers, proving that they are a great threat.
The fact that they all 'power down' when the mothership blows up is a bit of a cheat though.