They designed it to withhold the flood, or even eventually destroy them. The 'Halo' was designed as a weapon against them. I don't want to spoil the story so I'll stop there.
Search on google imgaes or look on the halo wiki. Their faces are never seen but thay appear tall with a similar build to a human.
An ancient race that built the Halo rings and the Ark in order to starve the flood. It is revealed by 343 Guilty Spark that humans are descendants of the Forerunners.
Random journal entrys from the forerunners
The forerunners built it even before our time and before the covenant.
A halo ring is a giant weapon designed by the forerunners to destroy all of the galaxy or in other words a big nuke.
Halo the game: 2003. Halo the ring: the time of the "Forerunners". HALO:CE( THE FIRST HALO GAME), ACTUALLY CAME OUT NOVEMBER 15, 2001. 2003 IS WHEN IT FIRST CAME OUT ON PC.
Halo 4, master chief lands on a forerunner planet. which awakens a new breed of enemy derived from the forerunners called, "Promethean"
Halo can't die. It is an custom made ring-planet built by the forerunners. So it can only break apart like the way it did in the end of Halo: Combat Evolved.
The Forerunners are an ancient race that made 343 Guilty Spark, Halo and the flood. When the flood started to reproduce dramatically in years before even Halo Wars (2531), the forerunners made mechanical things and oracles like Guilty Spark, and then killed everything on the planet, including themselves and the flood. All that's left of them are a few of their buildings, oracles and sentries, which try to kill you in halos 1-3.
When the forerunners built the Halo's, they built them with awesome powers. As the history of the halo go's, there are only eighteen in existence. After all the forerunners vanished, the halo's were lost in space, until over a thousand years later. The covenant found 1 ring and found the powers of it. They set a devoted there lives to finding all of them and activating them to have the galaxy all to them.
You can build stuff in halo 3,halo 3 odst and halo reach.
All I know is that halo movie comes out next yeah (can't wait) along with halo 4 in the winter (also can't wait) and that bungie are sendin Microsoft and 343 indestries secret documents and that the planet master chief was heading to in the trailer is a forerunner planet and the forerunners created the halos to destroy the flood billions of years before and trains the first Spartans. Also halo 4 contains the halo reach multiplayer online and any kind of forge is not yet official.so yeah possible chief may come across forerunners and old Spartans which would be pretty cool. And the forerunners created the sentinels and used them to build there civilization then the flood invaded and they had to kill themselves to activate the halos and wipe the university whilst keeping two of every species alive . Hope this helps