A cohort is a group of people who share a common characteristic or experience within a defined period (e.g., are currently living, are exposed to a drug or vaccine or pollutant, or undergo a certain medical procedure).
A rift valley is a lowland region formed by the interaction of Earth's tectonic plates. This small rift valley has a typical formationâlong, narrow, and deep. It was formed by the Thingvellir rift, where the North American and Eurasian tectonic plates are tearing, or rifting, apart over a hotspot on the island of Iceland.
Rift Keystones (or RKs) are small objects that drop randomly or are awarded for completing all five bounties in a given Act in Adventure Mode. RKs have no function on their own and are only useful to open up a Nephalem Rift. As that is a very useful purpose, they are quite valuable objects.