The murder in 2367 of Ambassador K'Ehleyr by Duras aboard the Enterprise-D, which led Worf to kill Duras in vengeance and left their son Alexander in his care.
The death of K'Ehleyr occurs in the Next Generation episode Reunion. K'Ehleyr, half Klingon and half human, comes aboard with the dying Chancellor K'mpec, who asks Picard to judge the contest for the succession between Gowron and Duras. She also brings Alexander, the son Worf did not know he had.
Curious about why Worf accepted disgrace, K'Ehleyr searches the records of the Khitomer massacre and learns that the father of Duras was the true traitor. Duras finds out and attacks her in her quarters. Worf and Alexander find her dying.
Worf claims the right of vengeance, beams to Duras's ship and kills him in a duel with bat'leths, which leaves Gowron as the only candidate. Picard places a formal reprimand on Worf's record, and Alexander is sent to live with Worf's human parents on Earth.