Agents Of SHIELD: 8 Ways It Sets Up Marvel's Inhumans
8. Kree
The Kree, an ancient alien race, featured in Guardians of the Galaxy, but its Agents of SHIELD where they have been used as a major plot device.
Its revealed that Kree blood was the basis for the G.H. serums, including G.H. 325 which brought Coulson back from the dead. That was just the beginning though, as it all tied into the Inhuman arc.
The Kree blood drives Coulson to the brink of insanity, as he gets constant visions and writes all manner of equations and maps when taken over by it, all of which point to an ancient city.
When Lady Sif arrives on Earth it is because a Kree ship was seen landing there, and she fights it and it ends up revealing the origins of Inhumans: the Kree came to the planet hundreds of years ago, in order to genetically engineer human DNA to create a superior race they could use as soldiers in their war.
They left, believing they had been unsuccessful, but the latent superhuman power was passed down throughout generations, along with Terrigen Crystals that would cause these powers to be activated.
This backstory is very much a part of the Inhuman mythos, and the show has done a lot of the groundwork for the Inhuman history by introducing them and their role here.