True Tolerance
Very rare2.5%ObjectiveOptionalSingle-player
Rule a realm containing at least 10 cultures with 95% or more acceptance of your culture
Ten cultures at 95%+ acceptance with yours - and your own culture does NOT count towards the ten. The wiki start: High Chieftain Radomil of Lusatia in 867, Polabian culture. Slavic cultures begin at 50% mutual acceptance in that era, and Polabian skips the Staunch Traditionalists tradition that would drag acceptance growth. Expand across Slavic lands until 10+ foreign cultures live inside your realm, then push acceptance up patiently; an Administrative royal court type helps once you have one. A proper multi-generation project.