What happens next?

How to prevent litter box aggression
Add another litter box.
The main cause for litter box aggression in cats is too few litter boxes in the home. Simply adding another litter box can often dispel much of the tension that is causing your cat to act aggressively.

[i] When you add a new litter box, place the box in a lower room in your home. You could then create a new box in the attic or crawl space where your cat frequents and destroy the old box.
[ii] The general rule of thumb is that you should have one litter box per cat, plus one. For example, if you have two cats, you should try to have three litter boxes in your home.
[iii] You may also try adding a partially new litter box if you are worried about your cat getting into the litter box. Try adding the new litter only if the cat responds positively, or if you don't want to add another one.
[iv] Keeping your cat indoors most of the time will not necessarily alleviate the aggression. It's important to make sure to keep increasing or decreasing your litter boxes to prevent the aggression.
[ii]