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.
After handing Tiamat's heart to Samael there will be a very interesting cutscene. In it War regains his ability to transform into “Chaos”, the red demon form which you originally had at the start of the Apocalypse. After the cutscene you will unlock the trophy.
