End of an Era

The trigger is the date hitting 1452.12.1, and the only way to fail is your dynasty dying out or losing every title. Don't run it on its own - fold it into other achievement runs and speed-5 the remainder once you've exhausted a save. If you want maximum safety, the Rurikids in 1178 have over a hundred living dynasty members (which also pops Prolific on the spot) - that dynasty is not going extinct by accident. From 1178 it's 275 in-game years; from 867 nearly double, so pick your start date accordingly.