So, it turns out, everything, every single villain, doomsday scenario, evil AI, and alien genocide the protagonists have come across is all the work of a single foe--perhaps one they thought they'd defeated long ago, or one they've never even met before but they still have a connection to. But it's all been one big plot--perhaps worst of all, it might have all been a Xanatos Gambit, and all their hard-fought victories have in fact strengthened their ultimate adversary.
This is the method that does one of two things:
1) Turns parts of what has happened in an episodic series is in fact part of an Arc.
2) Takes Arcs, usually important Arcs, that have occured in a more serial series and reveals that they were all part of a Myth Arc
I.e.: it welds Arcs.