The Theranians seem to be a very diversified species, some appear as sterotypical grey aliens, other green skinned, or even vampiric creatures.
The Theranians have been observing Earth for hundreds of years, even creating a replica society on their world.
When a Russian astronaut landed on Thera, the aliens decided to use him as a carrier for a virus so that he would wipe out the human race. Humanity at its weakest would easily leave their planet open to conquest.
- Humankind Issue 001