Ride ruin past the rocks that you used to get to the drilling platforms initially, you should encounter some more of the demon riders and if you continue past them you will find a gorge that leads back to the area where you battled with the Ash Titan.

From there you have two different tunnels, take the one that doesn't lead to the arena where you reunited with ruin. You'll eventually come back to the Leviathan's Drift area and make sure to activate the vulgrim location (hard to believe you passed it up twice?!). Ride ruin behind where the stygian was being held captive activate mask of shadows and shadow jump up the cliff.

The portal back to Eden is at the end of the path.