The novel really has two endings, Marlow’s story and the frame narrative. Marlowe trails off in the middle of explaining why he lied to Kurtz’ phrase of “it would have been too dark...” the frame narrative ends similarly. After Marlowe trails off the crew of Nellie sits quietly as the narrator looks towards London, which appears to be at “the heart of immense darkness”. Both endings make reference to “darkness”. The first reference being kurtz’ demise, the second being the ominous clouds over London. The novel reverses it’s idea of the Congo being the heart of darkness and instead makes London the heart of darkness. Marlow finds less of African savagery and instead finds European evil. Idk if this helps but I hope it does