Apocalypse (Eb Saban Nur), the X-Men villain received a glorious on screen introduction in the end credits of [X-Men: Days Of Future Past](movie:203942). In the scene we see a young Apocalypse constructing a magnificent pyramid while a multitude of people chant his name. Four horsemen look on as Apocalypse completes the pyramid. All this takes place in ancient Egypt some 5000 years in the past.
Overtime Apocalypse hibernates and is usually awoken thanks to some characters who work willingly/unknowingly to raise him.
Mystique will likely play a role in awakening Apocalypse from hibernation. In X-Men:Days of Future Past, Mystique was busy on her missions freeing mutants on her own. Its very unlikely she will be under Magneto in X-Men:Apocalypse. This may give her freedom to explore and may end up being involved with Eb Saban Nur(Apocalypse) or his agents.
Mystique has helped Apocalypse in other media and providing herself with security was a major motivation. However the Mystique we have met in the films has been more selfless than selfish. If Mystique will cooperate with Apocalypse it will most likely be to "help her mutant brothers and sisters".
Apocalypse may use this mutant as a conduit to interact with the world if Mesmero makes his debut in film in X-Men:Apocalypse. Mesmero has been known to use his mind-control abilities to influence others to do his bidding in this case Apocalypse's bidding. He did so in X-Men Evolution series.
This is another mutant who is yet to be used in film. I think Sinister is more likely to plot to bring the age of Apocalypse due to his desire to take mutation to the next level. In his desire, Sinister may plot to use Apocalypse's technology/knowledge to to do just that. But Apocalypse in not known to work under anyone (He was worshiped as a god in ancient Egypt) and if Sinister is involved, he might be Apocalypse's henchman.
The group consists of Death, Famine, Pestilence and War most of whom are mutants such as Angel (Archangel after enhancements). Apocalypse usually enters hibernation to replenish his energy and in some incarnations, the four Apocalyptic horsemen watch over Apocalypse's tomb. Sometimes it has been his own horsemen who have awoken him if it was the right time.
Apocalyspe at one time brainwashed Xavier,Magneto,Mystique to be his horsemen. This may be interesting to see in [X-Men: Apocalypse](movie:1194267).