A cursory examination of the plot of this classic Gothic tale reveals the many Gothic elements that it contains. If I had to pick four main events that could be considered the most visually Gothic, however, I would have to pick the following events.
1) Let us remember that Matilda uses magic to help Ambrosio seduce Antonia, his innocent and beautiful pupil. The use of dark sorcery is a key Gothic characteristic.
2) Matilda is revealed as an instrument of Satan, showing that she is purely evil, which is another classic Gothic characteristic.
3) Lorenzo's sister being tortured by hypocritical nuns would be another classic example of Gothic violence and excess.
4) Lastly, the final scene when the devil prevents Ambrosio's repentance and he dies a terrible death finding out that Antonia was his sister is also one of the most Gothic scenes in the entire novel.