Tyranids. It's the combination of the Flood, the Zerg and the Borg. Gestaltic hive-mind intelligence, every single member is psychic, rapid and directed evolution that constantly adapts to your weapons, tactics and even your genetic structure. What one member sees, the entire collective knows.
And when they swarm you and yoy get steamrolled, they take what is useful in your biological makeup and incorporate it. The rest of you is turned into organic paste which is then used to grow more of them. They devour entire planets free of any and all useful materials.
They are
hunger incarnate.