In my version the Abyss is connected to the Wilderlands. The gateway guarded by a magical ward created by the gods and the wards were protected by permanent hurricane/maelstrom.
The "last" campaign involved signs and portents that not all was right with the wards. (like fireballs becoming 10 times more effective and then returning to normal) The adventures got harder and tension grew. Finally the ward blew allowing the demons free access to the Wilderlands.
The players were about 15th level or so and I used battlesystem as a background for a big epic battle between the PC forces and the demon.s Great fun was had by all but unfortunately they fought just a scouting party.
Aside: Back then If you bought everything for Battlesystem 1st edition you wound up with a ton of dragon counters because of Dragonlance's Battlesystem module.
So after that initial battle was over the PCs were preparing for the main force. I described the roar of the winds generated by thousands of beating wings. I then proceeded to line the edge of the table with the three dozen dragon counters I had.
The players took one look and ran.
They had fun because they knew the Apocalypse was coming and the world was going to end. They just didn't know how it would play out They were really impressed with the line of evil dragon laying waste to the land.
The campaign world I designed to replace the Wilderlands was not as cool and so I returned to the Wilderlands for the next campaign retconing the Apocalypse out.
Recently I did run a sequel using 4e where the players were catapulted into that version of the Wilderlands. It didn't get completed unfortunately. The players quickly lost interest in D&D 4e and we switched back to using the other 4e, GURPS.
My idea was to use the Tiers system of 4e to divide the campaign into distinct phases. Heroic was in my normal Wilderlands, Paragon will see the players thrown into the alternate Wilderlands to deal with the post Apocalyptic world.
If they succeed with their Paragon Quest then they will find a way to go back into time to deal with the demons before the Apocalypse. In ordre to try to prevent the Apocalypse from happening. I thought the high fantasy focus of 4e was ideal for this type of campaign. I may still yet try this on another group.
A plot like this can work with the sandbox fantasy style. The main issue is throwing the players into the alternate timeline. I The main villain of the heroic phase was a leader of a demonic cult and he was setting up a ritual to change the time line.
Now to ensure that this event happened, you need to cover the possibilities
- The PCs bag the villain before the ritual starts
- The PCs bag the villain during the ritual
- The villian completes the ritual before the PC bag him.
#2 is covered by the fact the ritual backlashes and the PCs are thrown into the alternate timeline.
#3 is dealt with the PCs finding items that will protected them from a successful ritual. It will protect them by causing to exist in newly created alternate timeline. They won't know what they are exaclty for other than they help protect against the cultists.
As for the transition from Paragon to Epic. The desire to escape the demon haunted Wilderlands should be more than enough to propel the players to travelling back into time to prevent it from happening in the first place.
If all this seems to game-like then D&D 4e makes it easy to start at any level. So you can start the campaign at Paragon with the players thrown into the alternate world.
The ideals of Sandbox Fantasy are just that ideals. When applied to the game table you need to be pratical about how you acutally apply them. Players don't like to be railroaded all the time. But if it is cool enough you can get it away with it as a interesting plot twist as long as it is plausible given the premises.
I learned to hone this while running fantasy LARPS. There time and manpower limitation force use to use this technique a great deal many times. I quickly learned what kinds of plots players would have fun with and which they wouldn't.
I don't do this all the time in table-top sessions but it is a nice tool to have in your GM Kit to keep your campaign interesting.