The Forbidden Rites event league went live on 4 September alongside patch 0.5.5. Ritual encounters now appear through the campaign, the Wildwood returns, and the Trial of Chaos has been reworked.
What a new league means for buying currency is simpler than the patch notes suggest: the economy starts from nothing. Every Divine and Exalted Orb in circulation was found after 4 September, which is why rates move faster in the first two weeks of a league than at any other point, and why a price quoted at league start is close to meaningless by week three. Every orb is explained in our currency guide.
Forbidden Rites runs until the 1.0 release on 11 December, when it ends and the economy resets again. What 1.0 changes is in our release-date post.