Worf probably forgot to put him in his will

No. It's where true heroes go to fight an eternal battle.

Gre'thor is their version of hell.

From memory alpha : “Sto-vo-kor (also written Sto'Vo'Kor, or Suto'vo'qor in the Klingon language) was, in Klingon mythology, the afterlife for the honored dead, where all true warriors went after they died to fight an eternal battle against great enemies. The halls of Sto-vo-kor were said to be guarded by Kahless the Unforgettable.”