Oh! dear, that's awful. Isn't there some way you can keep the other cat out of the cellar. Could you possibly obtain a wire meshed covered, rabbit or chicken run, for the garden, so that your cat could have some freedom outside, without the other cat being able to get at him. Another possibility is if you have wooden fencing around the garden, you can buy Lion poo from Zoo's, over the internet. This you can put in a bucket with water, stir until it is liquid, and then brush along the tops of the fences (all round the garden). It won't smell strong to humans, but it certainly will frighten the other cat away, and stop it from coming over the fence. Failing these two suggestions, I don't know what else to think. Will keep trying to think of something.