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. (DIS: "The Vulcan Hello")
The Black Fleet was first introduced in the 1984TOS novel The Final Reflection. When a Klingon warrior dies an honorable death, he will serve in the Black Fleet, where he can fight and kill worthy foes who also have entered the ranks of the dead for all eternity. The book predates the introduction of the concept of Sto-vo-kor, and presented the Black Fleet as an alternative afterlife concept. However, since Sto-vo-kor was mentioned by T'Kuvma in "Battle at the Binary Stars", in canon the two concepts must be part of the same belief system.