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The biggest thing you have to be careful with in something like this is maintaining at least the illusion that what the characters does matters.
If you decide that no matter what happens, the fight is going to end with being taken into the future, it is really easy for the players to decide that they aren't interested in the story, since it isn't THEIR story, it is yours and they are just along for the ride.
Similar things can be said of 'They're gonna find the artifact and inadvertently release a powerful devil' if it doesn't matter what they actually do, no matter what the devil is getting released by them, then they are merely props for your story telling, not the ones driving the plot forward.
There isn't necessarily anything wrong with making sure you know how the story is going to go next, but for it to be an enjoyable experience you need to conceal from the players that whatever they do, X is going to happen. That can mean multiple ways of bringing about X, depending on what the characters do, and things like making it obvious that the bad guys are angry that their plans were disrupted, and they had to go with their less effective backup plan (what you really intended to have happen all along).
Having a predetermined plot and giving an illusion of free will is one of the more difficult things to do as a GM. Most often it fails and the players feel 'railroaded' so be aware that this is a difficult thing to pull off.