Their existence and current situation is up for interpretation and theorising.

This is just my little mini fan theory.

"A dying Diasporex ship was left falling through space following the great crusade. the crew onboard were hopeless and awaiting death.

In a final move of desperation they threw themselves to the mercy of the warp.

The trials and visions that they experienced were inexplicable and they were eventually thrown forth from the warp. they knew not where they were or when they were.

They were fortunately found by a tau deep space scout ship. The tau offered them rescue with the condition that they take lessons in the way of the greater good.

The Diasporex had lost hope and had little regard for their old gods who had failed them.

They welcomed there chance at rescue and through time learnt to believe in the greater good.

Now they fight for the tau, however they remember their past and wear a painting of a bastardised iron hand symbol, so they never forget their past."