Imagine and write the end of the story.

As the story ends, Roderick and Madeline die and the narrator flees from the house. He watches as it cracks and sinks into the tarn.
Think about how you would like to approach adding to this ending. One possible way would be to consider shifting the point of view. Right now, the narrator is watching from a distance as the house collapses and sinks away into the water. You might write a new ending in which you imagine being still in the house. Maybe Roderick and Madeline are not really dead and awake to find themselves in a disintegrating, sinking mansion. What do they see, feel, think, and do? What sounds do they hear as the house sinks? What do they smell?
Another approach would be to extend the ending of the story out past the point when the narrator watches the house disappear. You might imagine him heading for the nearest town and alerting people about what has happened. What occurs then? Does a group go back to the tarn and try to fish the bodies of the dead out of the water? What else might happen?
These are just two ideas that I hope will get your imagination going so that you can write a great ending to the story.