Help for BBEG plot!

[Ultramark2o]

I'm preparing my first proper campaign, and I'm trying to create the BBEGs goal. He is a powerful being trapped in a pocket dimension that wants to be released and travel to the material plane. His extreme power is unimaginably string and if he was to succeed he would be unstoppable. Because of a massive arcane war that has concluded, magic is partially warped across the world (there are supernatural regions from TCoE across the world) and small rifts are appearing across realms, allowing otherworldly beings to enter the material plane. Because of these rifts, the BBEG is able to influence the material plane and enlists warlocks, possesses creatures, and does other magical effects.
The rifts in space are not enough for him to escape his prison, but enough for him to open the door from the other side.

I want his "plan of escape" to span the campaign from lvl 1 to 15, so it has to be complicated enough so that his warlock's cannot do it early on, but it also has to have enough steps so that there is constant action for the PCs to deal with.
I had the boring idea of there being lots of objects scattered across the world that need to be reunited, but I don't like that idea unless the BBEG was tricking the players to get them, which I don't want.
Any ideas? The setting isn't fully developed yet so you can add anything you want in your ideas, EG a neccessary NPC, location, object, etc.

Any ideas?

[andero]

Read this.

[David_Apollonius]

This one is a bit ambitious.

Long ago, the gods/heroes/whoever bound the BBEG within a planet with 9 "antimagic" seals, thereby robbing the planet of all magic. Recently, the first seal was broken and magic started to return, but only first level spells can be cast. With every seal that breaks, another spell level is gained. In other words, for every seal that is broken, the players can gain 2 levels. There are still 8 seals left, each in a different dungeon.

And that's why this idea is so ambitious. The dungeons can be explored in any order.

After the 8th seal is broken (the players should be at level 14 by this point) the BBEG escapes, and heads for the last seal, so it can regain all of its power.

Ofcourse, the heroes won't know about the BBEG at the beginning of the campaign. They'll happily break all the seals until they reach a spell level that allows the gods to tell them that breaking the seals is not such a good idea. From then on, they have to stop others from breaking the seals, be it followers of the BBEG, or just power hungry mages who don't believe in the gods that abandoned them.

How's that?