Cycle of Nemesis — Kenneth Bulmer

Seven thousand years ago one of Earth's earliest civilisations was confronted by a menace from the stars - and died in a world-shaking effort to destroy that cosmic monster. But the death of that forgotten empire was not in vain, for they did succeed in entombing that dimension-shaking thing out of sight and harm to humanity.

But even their efforts could not make that burial permanent - and after seven millennia the monster stirred again, cracking through the new world of today.