I wouldn't use a Seal except for two reasons (as a GM):
1. The Seal is an essential part of the story, probably close to The Central Focus.
2. I need to juice up a boring world. Say you've managed to defeat all foes, institute just government, and put your world on a rising curve of economic prosperity....which is all great. But it can be boring for the player. Pop open a Seal, and unleash Chaos.
I had this situation in The Postman world. Early version Tad was President of the Reformed USA, daily we were expanding, technology was getting refitted, justice was all about....and my people both loved and feared me (so even those few who even in a good society have a hate on would be frightened of taking on the guy they called Ghost.)
I could have very possibly stayed a century there, and ushered in flights to the Outer Solar System.
Instead I started taking long trips in winter on horseback, and got frozen, and versed out.
Going back to #1:
1. The Central Focus: The Six Seals of Dread wall out the power of Evil since this is a world that had been overrun by Evil (Vampires, Demons, Dragons...). Now someone is running around trying to open the Seals. And the hero has to stop them before the waiting Army of Evil comes marching in.
2. Magic Blooms Suddenly. Yesterday was normal. Flew the jet to London. Today, well, we had to have jet fighter escorts to shoot down the wild dragons that rose to fight what they evidently thought was a challenging silver dragon as the jet flew home.
3. Magic Blooms in End of the World fashion: Things go nuts. Magic hits the world with roughly the same benevolence that an asteroid the size of Texas would. Society comes flying apart. Frequently, in this situation, a reverse Seal kicks in. That is, all Tech is now suppressed by Magic. Of course, this kind of requires that the magic bias stay high for that to work. You have mass die-offs, and millions fleeing the city, and small groups of Christians, and small circles of Wiccan covens finding to their surprise that no longer are they the little guys who are tolerated, but that they are rapidly heading toward being the Elephant. (of course, along the way are many who are not going to like that.)
4. The Apocalypse or Ragnarok: It is the End of the World. And the various Lords of Light and Demon Princes, heroes and monsters, have all come on stage to play their roles. In a few years from now, the final curtain falls. Or there could be some sort of wait like the Millenium Period in the Bible where Jesus rules in perfect justice, but in the end, Mankind reveals that at least a fair part of them are not that interested in justice or righteousness, and the last battle occurs.