They've known where the temple is for several centuries at this point. They spent centuries before that searching for it, going from place to place, canvassing the world, looking for the one weak point in the seal. The one fault their father had told them they would leave, just in case.
In case of what, they still don't know. That wouldn't have stopped them from using it for their own reasons.
Only, the council was clever. Or, their father was, for it was him who prevented them from breaking the seal themselves. Six of one, half a dozen of the other. The Artefact holding the seal shut is the worst abomination the council had created, but they might still have taken it if it could free them. However, as the wielder of a Complex Artefact of their own, the last one awake, and with their life preserved by the Lesser Artefact they wear...
They can't touch the Staff of the Magus.
And they can't lead just anyone into the temple to do it for them. The wards provided by the temple's Artefacts, some mix of Life and Illusion magic, prevents the entry of any who carry the plague. Something they couldn't possibly know until they brought them into the temple, where they and their magic would be in the same plane.
They do not think this could possibly take that much time. Surely the plague has died in the millenia since it nearly drove humanity extinct?