The purpose of the Shield Worlds was offensive measures. Although they protect something, the shield worlds were meant to be used against the Flood. The Didact himself wanted to use them instead of the Halo Arrays.
The Shield Worlds pretty much protect anything inside them from the Halo Pulse Effect. Requiem has for example squid-like flying creatures on the planet. They most probably survived the Flood-Forerunner War.
The Shield World 0459 (in Halo Wars) contains a massive fleet of Forerunner Ships. Considering that the Didact wanted to use the Shield Worlds as offensive instruments and the fact that SW 0459 contained a Forerunner Fleet, it is possible that Requiem also contains some sort of weapon.
If we look at this quote now
> But if something were to go wrong with one of the Halos, if our tools were ever turned against us … long plans, indeed.
we have a clue at the story of the Reclaimer Trilogy, especially when looking at this concept art.
A Halo turned against a Shield World? Who could do that? The Master Builder vs. the Didact maybe. The Didact wanted to use the Shield Worlds in the war against the Flood, the Master Builder and his group of specialists succeeded in building the Halo Arrays, however.
I do not believe that any of them will be the Ancient Evil. Nor am I sure that both will appear at all. Looking at this statement, however, gives yet another clue.
The notorious “unbound state”. Is it possible that a species or class-free Didact will fight something that tries to use the Halos against his Shield Worlds?
Whatever we will fight, I am sure it will include the Prometheans, all Shield Worlds and all Halo Arrays. I am also sure that the Ancient Evil will ‘turn the tools against their creators’. Requiem is just the beginning. It certainly hides something. Maybe a weapon, maybe a fleet, maybe the Ancient Evil.
