From Destinypedia, the Destiny wiki
Sacrilege is the tenth story mission of Destiny 2, taking place between Six and Fury. It is the first mission on Io and completion of this mission will unlock the Io Adventures Unexpected Guests and Road Rage.
During the course of Sacrilege the player locates Ikora Rey and is tasked with stopping Red Legion activities on Io. These activities are quickly discovered to be the theft of the Traveler's energy still remaining in Io since it was incompletely terraformed by the Traveler at the end of the Golden Age and onset of the Collapse. Ghaul, Ikora believes, intends to use this raw Light based energy to perform the equivalent to Taking on the Cabal and make himself immortal in the absence of the Traveler's wilfully blessing the Cabal with the Light. During the course of this discovery being made the player is ambushed by the Taken, who, though apparently leaderless, have been drawn like moths to the fire to the gigantic pillar of Light which shot into the air when the Cabal mining operations struck Light, or more accurately the residual energy on Io, which they attempt to steal, and the Taken attempt to invade and consume. The player manages to stop this Taken invasion for the moment, after they kill Irausk, the Herald of Savathûn, which, together with Ghost Scans throughout this mission suggest the Taken are not as leaderless as Ikora thinks.