In Greek mythology, Lethe was both a river in the underworld and the Greek goddess of forgetfulness and oblivion. There was also a character named Lethe in Dagger of the Mind (1966) and a telepathic species called the Letheans in Distant Voices (1995) and The Sword of Kahless (1995).
The biological explosive that the Vulcan extremist uses to try to assassinate Sarek was first used in Star Trek: Enterprise season 3, episode 12 "Chosen Realm" by the religious extremists to take over the Enterprise.
After Sarek in The Vulcan Hello (2017) and Harcourt Fenton Mudd in Choose Your Pain (2017), Amanda Grayson is the third character introduced in Star Trek (1966) to appear in Star Trek: Discovery (2017).
To induce and enhance a telepathic connection between Michael and Sarek, the device that is attached to Burnham's face places sensors on three key points: her temple, her upper cheek, and her chin. These points line up with the key points that Leonard Nimoy selected when developing the traditional Vulcan mind meld technique.
This episode takes place in the 2240s and December 2256.