The Forerunners were a race that was only ever really hinted at in the many stories of the Halo games and if any players wanted to learn more, they really had to go out of their way to find out about this mysterious race that came before.
That was before Halo 4, which introduced the last surviving Forerunner, the Didact and his army of Forerunner drones that he was going to use to accomplish his goal of killing humanity. Now that we've seen them more, let's go over a few things about the Forerunners.
10 Origins
The Forerunners, despite what their name implies, are not the first race of intelligent beings in the Halo universe, even during their own time. The Forerunners were created by an even older race of beings known as the Precursors that seeded the Milky Way galaxy with life for millions of years. The Forerunners were seeded on the planet lived on Ghibalb, here they continued to evolve to eventually take the place of the Precursors in the universe.
9 Rebellion
Originally, the Forerunners were chosen by the Precursors to take over their place and the Mantle of Responsibility (we'll get into it) however, The Precursors eventually changed their minds, instead choosing the newly seeded and evolving race of what would become humans. This changing of the guard also caused the Precursors to label the Forerunners as problematic and attempt to wipe them out and the Forerunners rebelled, killing most of the Precursors.
8 Weird Names
Stepping away from the creation of life and mass genocide on a galactic scale for a bit, let's talk about how weird the Forerunner names are. The Halo universe is no stranger to strange names, whether it being weird alien names like Thel'Vadam or weird grandiose ones like Prophet of Truth, but the forerunners take it up a few notches. The Forerunners have names that are more like space-based phrases, for example, the Didact's name is actually Shadow-of-Sundered-Star.
7 Mutations
From the time when a Forerunner is born to their adolescents before reaching maturity, they are called Manipulars. Forerunner puberty is a little different than the one humans go through, instead of just getting taller and maybe getting facial hair, Forerunners grow entire new appendages based on a caste that they choose to join.
These additions range from increased size and mass to additional limbs to any number of things. This process is also artificially induced, meaning that the Forerunners came up with this.
6 Caste System
On the topic of the caste system in the Forerunner society, the caste system called the rate system by the Forerunners, is what dictated a Forerunner's role in life and how their physical body would change. While most young Forerunners could choose whatever rate they wanted to join, they were also generally expected to join whatever one their family tended to be in. There were rates such as Builders, Miners, Warrior-Servants, Lifeworks and Engineers.
5 Wore Armor All The Time
The Forerunners relied heavily on their suits of armor, no matter what rate they belonged to. These suits of armor had many benefits on top of the level of protection that you would expect, including constant life support systems and medical care that indefinitely expanded their life spans and augmented their physiology and mental faculties.
This armor also provided Forerunners with the ability to live without ever requiring sleep, as well as entertainment for them.
4 They Really Liked Hats
Yeah, this is a bit of a weird one. Hats were one of the favorite things of the Forerunners and they were so beloved that headwear became a very important part of their culture. The headwear of each suit of Armor that each Forerunner had was a way to show which rate each Forerunner belonged to, except for on a special day in what was known as the Grand Star Season, where every Forerunner wore the same style of headwear.
3 Discovering The Flood
After defeating the Precursors and wiping out the original race of humans and scanning their DNA, the Forerunners encountered a race that would eventually become the terror of the entire galaxy known as the Flood. The Flood was first encountered by a Forerunner survey team on the planet Seaward, this was the first planet that was taken over by the Flood infection and the Forerunners instantly recognized the threat that this new enemy posed to the whole galaxy.
2 Vegetarians
As a part of taking over the Mantle of Responsibility from the Precursors, the Forerunners let it become the philosophy of which the vast majority of them lived their lives. This includes what the Forerunners called "the eating of the flesh of unfortunates" and that being forbidden, as it was considered an overly violent act to a weaker being. That is a bit ironic considering that the Forerunners had no problem wiping out the Precursors or the original humans who were basically just learning to build houses with sticks.
1 Mantle Of Responsibility
We've talked a lot about the Mantle of Responsibility, but just what is it and what does it teach and entail. Commonly referred to as just The Mantle, it is a philosophy started by the Precursors that said that whichever race bestowed with the Mantle will be charged with the protection and cultivation of the species and planetary systems within their empire or reach. Despite the goal of this philosophy is to preserve life, the Forerunners took it as preserving the DNA of all beings and then killing them.