In the short story "The Black Cat", why is the cat found in the wife's tomb?
The reason that the narrator murdered his wife in the first place-at least so he says-is that his wife kept him from swinging an axe at that very cat to kill it. Enraged, he says, "I withdrew my arm from her grasp and buried the axe in her brain". So, what was meant to be a blow at ending the cat's life, ended his wife's life. The cat had been tormenting him and getting on his nerves for quite some time, so when it had followed him into the cellar and nearly tripped him, he was going to let him have it. Well, after he kills his wife the cat flees and isn't seen again, until he discoveres it in his wife's tomb.
Why is it there? Mainly for good storytelling. It is much like Poe's other story, "The Tell-Tale Heart" where a man buries his victim under the floorboards, but while the police are there, the muderer thinks he hears the victim's heartbeat so confesses. Here, it isn't a heartbeat, it is a cat's meowing and crying behind the wall. The cat could represent the conscience of the narrator, or even more likely, his come-uppance. It ensured the narrator's demise for the cruel behavior towards it, and the murder of the wife. Because it was in the tomb, it alerted the authorities to the location of the wife, and the murderer was had. So Poe put it there for symbolic reasons, and as a way to make a horrible person get caught at his awful deeds.