Part of the regular story progression. No special work necessary. Impossible to miss. Below is a detailed spoiler of the event that unlocks this trophy if you are still curious.
During your mission to slay the boss Stygian in the “Ashlands”, you will be thrust into a strange demon arena. In it you will face a demon horseman, that is riding none other than War's steed “Ruin”. After you defeat the horseman you will be, as the name of the trophy suggests, reunited with Ruin. Allowing you to ride him throughout the rest of the game (in designated areas), as a member of the Four Horseman should.
