- "... 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".
Once the Ritual was completed, the body was unceremoniously discarded. It was considered to be "only an empty shell" which should be treated as such. (TNG: "Heart of Glory")
Instances
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. In this instance, Worf opened the slain warrior's eyes. (DS9: "Tacking Into the Wind")
Other Klingon rituals concerning death
There is a Klingon mummification glyph, indicating that at some point in the past the Klingon mummified their dead. Spock identified this glyph during his mental retraining following his fal-tor-pan. (Star Trek IV: The Voyage Home)
Ak'voh, the act of watching over a fallen warrior's body, was an ancient Klingon tradition. (DS9: "The Ship")
R'uustai, or "The Bonding", is a rite of brotherhood sometimes performed after the death of an individual's parents. (TNG: "The Bonding")
A funeral was held for Dax, son of Kang in 2290. At his young age, it is unclear if Dax completed the Rite of Ascension, becoming a Klingon Warrior. (DS9: "Blood Oath"; TNG: "The Icarus Factor")
If an individual dies in a manner that does not ensure entry into Sto-vo-kor, his or her relatives may fight a great battle in the deceased's name; a victory will allow him or her to enter Sto-vo-kor. (DS9: "Image in the Sand", "Shadows and Symbols")
Apocrypha
In Klingon for the Galactic Traveler, the Klingon language translation of "death ritual" is given as "Heghtay".