Stulgid of the Great Ring was first referenced in an infographic in X-Men (5th series) #16. It was confirmed that Xilo and Stulgid were the same entity in S.W.O.R.D. (2nd series) #8.
Xilo acts as the living history of Arakko, storing knowledge within the worms that compose its form, adding worms as necessary for storage purposes. Death or destruction of the worms leads to the loss of historical record. The collective entity's personality grows and changes over time, to the point where it feels the need to re-name itself, such as from Stulgid to Xilo.
Marauders (2nd series) #12 showed Xilo among the mutants of Threshold in the Paleozoic Era, making them the longest-living known mutant today besides Krakoa and Arakko.