The Delphons were first introduced in the novel The Black Moon, although the name was not yet given. "Delphons" is a human designation, as the original name has been lost due to the species being extinct for over 3 million years. They were an aquatic humanoid species with a technological level of humanity in the 29th century.
Delphons were one of the four known sentient races to exist in the same region of the galaxy 3 million years ago. Despite them being an aquanic species, they created colonies and bases on many planets, including Earth.
Around that time, the Delphons became aware of the Forerunners - the first lifeform to ever exist in the galaxy, composed of proto-matter. They possessed an animal-level intelligence and moved as a herd. The Forerunners ate all forms of matter, as they slowly moved through the galaxy, consuming entire systems, save for the stars themselves. The four races detected the Forerunner horde moving towards their sector of space. Unlike the other three races, the Delphons chose to fight. However, there was only one way to destroy the proto-creatures - novae. As the Insects and the Logrians evacuated their worlds and left, the Delphons were priming their own stars to explode. As their hypersphere technology was not advanced enough for them to escape, they chose to stay and sacrifice their lives to try to stop the Forerunners from "eating" the galaxy. Whenever the horde entered one of their systems, the Delphons would activate the nova sequence. However, there were too many to stop with just one stellar explosion, so they did it again, and again... until all the Delphon systems were gone. However, nearly all Forerunners were also destroyed. The few that remained ended up in a coma-like state and stayed that way for 3 million years.
With their sacrifice, the Delphons wanted to give younger races (including Humanity) a chance to live. Nearly all trace of the Delphons was wiped out in the ancient cataclysm. However, the novel The Black Moon describes an archaeological expedition to one of the charred worlds that remain in the destroyed systems. There, they locate several dead Delphons, perfectly preserved in ice, and the remaining Forerunners. Due to the ambitions of the person in charge of the expedition, the cataclysm was almost repeated.
That same person wanted to know more about the frozen aliens, so he found a frozen human, killed during the First Galactic War. He downloaded the memories of one of the Delphons and put them in the human's brain, hoping that the brain will act as a decoder. Eventually, the man used the new knowledge in his brain to find a semi-sentient species dormant on one of the charred Delphon worlds, which he used to destroy the last of the Forerunners.
It is likely that he eventually passed the knowledge to the proper people, as the information is public in later novels.