I think the following best matches your described meaning: -
Star Wars is a story that has been elevated to cult status.
Star Wars is a story with a cult following.
Both of these sentences convey the meaning you've described of being able to inspire an almost fanatical fanbase. Some owing to the emotional attachment they place in the fictional worlds, actually merging their own reality with that of the book/film/franchise etc, by for example learning elvish as you've mentioned etc.
Cult status (dictionary.com)
A popular person or thing having strong enduring appeal and elevated
to worship by some
Cult following (Wikipedia)
A cult following is a group of fans who are highly dedicated to a work
of culture. A film, book, musical artist, television series or video
game, among other things, will be said to have a cult following when
it has a small but very passionate fanbase. A common component of cult
followings is the emotional attachment the fans have to the object of
the cult following, often identifying themselves and other fans as
members of a community. Cult followings are also commonly associated
with niche markets. Cult media are often associated with underground
culture, and are considered too eccentric or subversive to be
appreciated by the general public or to be commercially successful.
Many cult fans express a certain irony about their devotion.
Sometimes, these cult followings cross the border to camp followings.
Fans may become involved in a subculture of fandom, either via
conventions, online communities or through activities such as writing
series-related fiction, costume creation, replica prop and model
building, or creating their own audio or video productions from the
formats and characters.
The Wikipedia entry goes on to list both of the franchises you refer to in your very question: -
Franchises such as Star Trek, Star Wars, Doctor Who, Harry Potter, The
Lord of the Rings, Game of Thrones, Clueless and Mean Girls attract
mass audiences but also have core groups of fanatical followers