Obtain the Horn of a Unicorn. These come from unicorns or rarely from Nightblood Invaders. Unicorns can be found at ( 300, -25, -66 ) /waypoint 300, -25, -66. Equestrielle the Corrupted can also drop it, so feel free to beat down this heroic unicorn.
Return and speak to Inara. She will take the corruption into herself to try to defeat it.