The Precursors are older than the universe itself. They have never existed as a single form, changing throughout time. Sometime they were a great spacefaring civilization, other times they were locked to a single planet, never developing the technology to leave it. They've even existed as sentient energy. Their technology was beyond advanced. Their structures and architecture utilized neural physics to bind everything together, making them indestructible to everything but a weapon that targets neural pathways for destruction. They had the capability to warp space-time, jam slipspace, and use a previously unknown FTL method called neural physics translocation. They utilized a sentient communication and information repository network called the Domain, one that was seemingly destroyed when Halo fired. They had the capability to to accelerate the evolution of intelligent life. It seems they are the creators of the Mantle of Responsibility. They created both the Forerunners and humanity, and seeded the Milky Way with life, as well as the Large Magellanic Cloud. Are there any still alive? This is spoiler territory, so be warned. [spoiler]Yes. While many of them were wiped out by the Forerunners, some survivors left the galaxy to flee beyond the reach of the Forerunners. The Primordials are all extinct now, the last one having died during the Forerunner-Flood War. Currently, the most prominent Precursors left are the Flood. Curiously, both the Flood and the Primordials were Gravemineds, making some interesting implications for the nature of the Precursors.[/spoiler] All knowledge the UNSC has on them is present in the Forerunner Saga. The events of Cryptum were attained through the Bronstellar Relation found on Shield 0006, the events of Primordium through 343 Guilty Spark, and Silentium through the corpse of a Juridical found on Shield 0006.

Woah, so they're the Lovecraftian side of Halo

Without a doubt. They are cosmic horror incarnate.

Also, on a side note. Having recently discovered that there was still a bunch of Spartans on Reach while Halo: Combat Evolved took place, I was wondering what you think about the fact that we are going to be able to play as Blue Team in Halo 5. I am personally really excited since Linda is my favorite Spartan II