I assume you are talking about the revenge that Injun Joe talks about in Chapters 26 and 27. If that is what you are talking about, then Injun Joe is planning to take revenge against the Widow Douglas.
Huck and Tom hear Injun Joe talking about revenge and they worry that he is talking about them. But it turns out he has a grudge against the widow. It's not really against her, but against her late husband. He had Joe horsewhipped in public and Joe wants revenge. Since Mr. Douglas is dead, Joe will go after the widow instead.