Adam Warlock was the genetic creation of a group of scientists called Enclave. He was an army soldier prototype for world domination.while forming in his cocoon, Adam overheard his creators' plans. When he hatched, he rebelled, destroyed the Enclave base, and left.
Soul gem introduction
The soul gem was given to Adam by the High Evolutionary. With this, he can heal his soul after a fierce battle, mostly against Thanos.
We did see him in Guardians of the Galaxy
Remember the yellow cocoon-like thing in the Collector's place: that's Adam. It was confirmed by James Gunn.
We could see him in Guardians of the Galaxy 2
Adam Warlock could be introduced in the Guardians sequel to introduce the soul gem. He would team-up with the Guardians to either protect or search for the gem.