Donald Wandrei: Raiders of the Universes
They were unstoppable until they met the humans

It was the 34th century and all five of the Federation of Planets around Sol were buzzing with their usual activity when the Raiders appeared. They were indeed Raiders of Universes because they had ravaged many systems before reaching Earth and showed no signs of slowing down in the least. Their weapons were invincible, their greed merciless and their natures completely alien. Indeed 'they' were from another dimension entirely. Eating up entire solar systems and planets, they slowed down just a bit when intelligent life was found on Earth. Not much since they were in a hurry, but just long enough to tell the Earthlings to mine all the Radium ore on Earth and put it conveniently on the surface ...... in six days....or else. -