Here is how Apocalypse and the Horsemen join forces with the Villains in War of the Apocalypse.
(we see the Villains in a meeting)
Shredder: At last, we can destroy them!
Myotismon:(senses something) We might be having some unexpected guests.
Kurumi Tokisaki: What are you talking about?
(as if to answer her question, everyone turned to see what looked like a blue force field appear and disappear)
(inside the field was none other than Apocalypse and his Horsemen)
Klorgbane: Why are you people here?
Apocalypse: To forge an alliance.
Bowser Jr.: Why should we trust you?
Apocalypse: My followers have abilities you do not. For example, Erik can move the very Earth with his powers.
Magneto:(demonstrates by destroying a mutant concentration camp)
Puppetmon: I'm still not convinced.
Apocalypse: I have intentions of rebuilding my pyramid in Tokyo, so I can transfer my consciousness into this boy.
Archangel:(reveals he is holding Hiatt Grey)
Apocalypse: During the transference, I will be vulnerable. I will need protection.
MetalSeadramon: That's a bit overkill.
Piedmon: Talk about turning a hero to the Dark Side.
Nightmare Moon: I am impressed.
Bowser: Very well, but not because you were going to Tokyo anyways, but because you have impressed us all.
Apocalypse: Wise choice.(looks at Megatron and talks to him, via telepathy) You never should have visited what was left of Sokovia that day.
Megatron:(in his mind) And why is that?
Apocalypse:(in his mind) Because I sense two minds in you.