Recently, some friends of mine and I were discussing a possible vocabulary for playing D&D in Latin. I suggested that the names of spells would have to be declinable if you’re going to have a ‘scroll of’ or ‘potion of’ that spell. It was pointed out to me that of course in English you don’t have to do this. You can just take the name as it is and use it as a noun. Of course, in English, this generally doesn’t make much of a difference – there aren’t many accusative markers left in the language. But we don’t listen to The Whom. We don’t watch Dr. Whom.
What is the rule in Latin for just using a title in as if it were an indeclinable noun?