Complex sentence: Tonight, the Old Man tells the Boy, is the anniversary of his mother's wedding night, the night on which he was conceived after a night of drunken carousing by his father and thus when his mother's fate was sealed.
Split into simpler sentences: Tonight, the Old Man tells the Boy, is the anniversary of his mother's wedding night. This was the night on which he was conceived after a bout of drunken carousing by his father, and thus when his mother's fate was sealed.