a 3d video game with a vast world, with many realms. Each realm has a race of people, who are either helped by, or under the tyranny of some flying demon. Each realm also has a "Flying Demon", whom you can ride, but only if you solve some riddle, or defeat them in battle, or solve some other puzzle.
from that point forward you can summon a flying demon to ride. Each one will have some unique ability, such as speed, stamina, altitude, tight handling, transportation, transformation, and so on.
There is an over arching goal, in that the entire world is under threat by a species of what may be called "Gaia parasites", who feed directly off the planet's life energy. As you travel the planet, the previous realms you encountered may become infected, and mutate the creatures there. The goal would be to revisit every realm, and find the solution to this global menace.
why were you traveling in the first place? You are an alien, and you are trying to find the uberdemon that can travel between planets, so you can go home.