- "... and as I watched Worf, it was like looking at a man I had never known."
The Klingon Death Ritual was a Klingon rite performed during, or directly following, the death of a warrior.
The Ritual involved opening and staring into the eyes of the dying individual, then bellowing loudly at the sky. The latter served as a warning to the dead (presumably in Sto-vo-kor): "Beware, a Klingon warrior is about to arrive".
It was extremely rare for non-Klingons to witness the Klingon Death Ritual. The first such case was believed to be in 2364 when Korris, Konmel, and Worf performed the rite, following the death of Kunivas. Soon after, Worf performed the Ritual for Korris. (TNG: "Heart of Glory")
Worf performed the Ritual again in 2367, following the death of his mate, K'Ehleyr, and in 2374 for his wife, Jadzia Dax. Though, in both cases, Worf did not open the eyes of the fallen warrior. (TNG: "Reunion"; DS9: "Tears of the Prophets")
Worf was last seen performing the Ritual in 2375, after slaying Gowron in personal combat. The fact that Worf opens the slain warrior's eyes in this instance would seem to suggest that this aspect of the ritual is unnecessary or inappropriate for a fallen mate, or that Worf could not bear to open the eyes of K'Ehleyr and Jadzia due to his attachment to them. (DS9: "Tacking Into the Wind")