My favorite is Eldrazi - a race from the table game "Magic: The Gathering".
It's a swarm of formless creatures, who came from an alien dimension to devour life from everything they see. They have three titans - Ulamog, Kozilek and Emrakul, who lead three kinds of creatures: processors, who turn everything alive into energy for Eldrazi, wastelanders - who own colorless deserts left after Eldrazi, and shapeshifters, who infect and change every living creature into Eldrazi monsters.
According to the MTG lore, it's impossible to kill the titans, and magicians-protectors are only able to imprison them for some time.
There are hundreds of different Eldrazi, so if you feel interested, you just go to MTG databases, for me it's impossible to describe the whole scale of the race in just one message.
I didn't play MTG for a while, but as far as I know, Eldrazi are banned from all the championships now, because of disbalancing power, and, also, they are hated by all the players for very strange and very complex tactics, which is too difficult to overcome, just because you don't understand how it works. I, however, is a huge fan of Eldrazi, for two years been playing only them. Always reminds me Lovecraft's ostraconophobic horrors