What happened to Tom's forehead?
In "The Devil and Tom Walker," Tom's forehead is marked by a print, like that of a burned finger, and the mark cannot be removed. This happens after he meets a stranger while walking through the swamp. Tom quickly recognizes the stranger as Old Scratch, or the Devil, and the two men begin to talk. Specifically, the Devil tells Tom about some buried treasure which belonged to a pirate called Kidd. Being very materialistic, Tom is keen to know more.
During this conversation, the Devil offers to tell Tom the whereabouts of this treasure in return for something from Tom. Presumably, the Devil wants Tom's soul, but Tom is reluctant to make such a deal. In addition, Tom wants some proof that what the Devil has told him about the treasure is true, prompting the Devil to leave his signature. This signature is the burned finger mark on Tom's forehead.
The short story, "The Devil and Tom Walker," by Washington Irving, resembles the sixteenth century story of Faust, an astrologer, who sold his soul to the devil for money and power. In this same way, Tom Walker, who is a "miserly" (line 19) fellow and will do anything for money, even cheat his own wife, meets up with a swarthy man in the swamp who the reader later discovers is the devil or "Old Scratch" (120).
During Tom's conversation with Old Scratch, he is told that the he could have access to unlimited wealth that is under the Devil's control. Tom questions the truth of the Devil's story, and the Devil imprints his signature on Tom's forehead. This signature is the mark of the devil and binds Tom to the agreement that will ultimately lead to his fate: an eternal life in hell.
Tom asked the devil to provide proof that all he has promised Tom is true. The devil pressed his finger to Tom's forehead and told Tom, "There is my signature." When Tom got home, he realizd he had the imprint of a finger burnt upon his head.