The First Ones
"There are things in the Universe billions of years older than either of our races. They are vast, timeless, and if they are aware of us at all, it is as little more than ants and we have as much chance of communicating with them as an ant has with us. We know. We've tried and we've learned that we can either stay out from underfoot or be stepped on ...They are a mystery and I am both terrified and reassured to know that there are still wonders in the Universe. That we have not yet explained everything. Whatever they are, Miss Sakai, they walk near Sigma 957 and they must walk there .. alone."
- G'Kar to Sakai: "Mind War"
The First Ones are vast and timeless beings. They explored beyond the Rim, built empires and taught the younger races as they emerged. They also fought the Shadows. Over a million years, the First Ones slowly went away. Some passed beyond the stars, never to return, whilst others just disappeared. In the mid-13th century, some First Ones joined a coalition of worlds, including the Minbari, to defeat the Shadows in a Great War.
After that, most of the First Ones went away, although some remained, most notably in the region of Sigma 957.
The First Ones were tracked down with the help of the Great Machine on Epsilon 3. When they were asked for help in the coming war against the Shadows, they initially refused, but then, after persuasion from Marcus and Ivanova, they agreed.
"When it is time, come to this place. Call our name. We will be here."
- First Ones: "Voices of Authority"
These remaining First Ones, with their enormous and extremely advanced and powerful ships, were gathered together, after some persuasion, to help the Army of Light led by Sheridan and Delenn to defeat the opposing armies of the Vorlons and the Shadows in the Shadow War in 2261.
At the conclusion of the Shadow War, all the remaining First Ones, including the Vorlons and Shadows, departed beyond the Rim, together with Lorien, the original First One. Some of the First Ones' technology has remained: Clark's administration on Earth made use of Shadow technology, to create a new hybrid class of ship, while the Rangers' White Star Fleet is a combination of Minbari and Vorlon science. Now that the First Ones are gone for ever from the galaxy, to whatever lies beyond the Rim, they have left the younger races in charge and responsible for building the future.
As Lorien said to Delenn, Sheridan and the rest of the younger races at the end of the Shadow war:
"I waited a long time for someone to find me. Now, like the others, I find I hate to leave. But none of us can stay behind this time. That was why it was important to find all the remaining First Ones. This is yours now. And you have an obligation: to do as we have done, to teach the races that will follow you ... and when your time comes, as ours has, to step aside and allow them to grow into their own destiny. If your race survives, if you do not kill yourselves, I look forward to the day when your people join us beyond the Rim."
Lorien in Babylon 5: "Into the Fire".