What was the mark the Lord put on Cain?

There is one terrible figure one meets at times in the dreariest wastes—a man tall and powerful, half-naked, the skin of some animal being all his clothing. The hair hangs wildly about his temples—there is a furtive look in his eye and his brow is gloomy. There is a mark on his forehead and he carries a club—not that he seems to require it, for he is a fugitive always, in constant fear of being slain. Everyone who meets him trembles, but he is afraid of the weakest and most helpless of creatures, fleeing them each and all for fear of his wretched life. Always alone, he seems nowhere and everywhere. A cursed fugitive he was on earth—a cursed fugitive he is in hell, for the Lord has set His mark upon him, that everyone should know Cain, and not slay him.

Letters from Hell