It is critical to avoid a TPK that you do not let the PCs know that the Ob are planning anything big on Axis Island. They should just think it was a Danoran base with some research purposes. PCs will be stupid and charge the castle, and unless you want the campaign to end early, you really shouldn't let them get in and defeat all of the Ob's defenses.
Normally, in adventure 3 the party learns that there are these old seals that somehow link this world to others, but there's no hint of the axis seal. In adventure 4 they learn the Ob have designed a lantern that makes transitions between planes easier. Adventure 5 reveals the colossus, though its purpose is a mystery. Adventure 7 has the Ob reveal their goal - to link to different worlds than the current ones and thus change the fabric of reality - but they specifically don't mention the Axis Seal in order to avoid spies finding out. Adventure 8 has a misdirect to make the party think Flint is the focus of the Ob's ritual; it's only one component, not the main one.
Only at the end of Adventure 9 do the PCs learn that the Axis Seal exists.
But as early as adventure 3 the party can get an inkling that the world use to be getting messed up by extraplanar invaders, and the seals helped limit who could come or go.