Black Fleet
The Black Fleet was a concept related to the Klingon afterlife. 

This fleet was mentioned in The Warrior's Anthem, in which it was said the departed honored Klingon warriors would join their fathers and fight an eternal battle. ()

After Rejac was killed, T'Kuvma expressed his hope that the ancestors, whose bodies formed the symbolic armor of his ship, would welcome Rejac into the Black Fleet. ()

See also 
 Barge of the Dead
 Sto-vo-kor

External link 

Qo'noS
Mythology
Death