Pretty much what it says on the tin.
Made up of several overmap tile hordes traveling together.
I think it would add a lot to the game. Currently the world feels very dead and sandboxy. It lays dormant at the players whim. The occasional, tiny “horde” aka group imho doesn’t pose much threat, even if the impossible happens and they wander into your base.
The mega hordes would add to the game, it would make the world more alive, it would force the player to react to something, finally which is a big problem for me - too often I find myself lost, wondering what I should do next and not seeing much reason why I should do anything. I want to survive. I want to be forced to run away or fight, to be forced into a corner and pull off an awesome, dangerous and bold escape that would make my blood start pumping. I want to wake up in the middle of the night whether inside my base or in my car and feel terror on realization that I am completely surrounded from every side. I want to be desperate, and a Mega Horde is about the only thing that would do that.
Swarms of ants, bees, Mega Forests and Mycus Waves are a bonus for variety. Maybe even a stampede of deer or something of that type.
Just imagine it. Opening up the map and instead of dully searching for a deep blue L or something, you feel the tension and worry about that giant blinking blob of Zs as they wonder dangerously close to you. And then a giant frog hops on a minefield next to you. You think nothing more than “ha, free meat” at first. But then you realize. Your pupils dilate. Your heart races. You open the map. You close it and pick up the meat. You open the map again and…
It has moved.