You have it in hand, the definitive beginning, the interesting and fascinating way the hero sets out on his journey, or perhaps the wonderful setting that he grows up in, with potential for inciting incidents, or the in media res moment that you think really should be the beginning, with everything else being relegated to backstory.
The downside is that you have nothing else. Not character (at least more than was revealed in the moment that inspired you), not more of the setting, nothing of the plot except what conflict is shown in the moment.
The upside of the downside is that you are legitimately as ignorant as the characters, and therefore can portray their wonder, frustration, and bafflement quite convincingly. And their mistakes. Not all of your mistakes can be incorporated as theirs, but it's a start.
Toward that, it may be wiser to start development with what the characters would already know. Set up that the beginning definitely implies exists, and may even require. Get the backstory straight.
Some are easier than others. If the idea starts with a character still as a small boy, learning about different kinds of spell-slingers, and how wizards and sorcerers and enchanters can get angry if you call them by the wrong term -- if the idea doesn't come with a pre-packaged location where he would have to learn that (such as an inn), you may have to consider where to put it, such as an inn, or a market that these people frequent, or an inn at a market.
If, on the other hand, the characters are fleeing through a forest, the questions are many. Who are these characters? What do they flee? Why do they flee it? Do they have a place in mind to go? Are their decisions wise?
But the second one has the real advantage: it has the inklings of a plot. Conflict is of course the life-blood of plot, and beginnings are much less likely to have it inherent than endings. (Not invariably. Some endings are of the resolution, not the climatic collision.)
The story for the first one could, of course, turn on an offended sorcerer causing a problem that the boy (hopefully, no longer small) must fix. But don't box yourself in too quickly. Obviously the differences must matter, but there are a lot of implications to them, which could lead to all sorts of conflicts. A too straightforward treatment risks the readers finding it dull and obvious.
This is not quite the danger of knowing the ending and heading straight toward it1. You are still filling out the effects in ignorance.
But, unlike the character, you can pounce on your first idea and actually make it the solution.
This produces plotting problems, in that the story lacks conflicts, and originality problems, as obvious ideas tend to be obvious to everyone.
It helps, of course, if you can throw other ideas in.2 If the original idea does not turn on what, exactly, the types of magic are, you can throw in your idea of magical singing.
There are a lot of issues. Making the magic too neatly diagrammed -- if it is orderly, it needs to rhetorically fleshed out so that it's clear that the order reflects a deep structure. Making the magic too grab-bag -- this enchanter sings spells, that pyromancer casts fire spells, this one does calculations first and then uses his wand -- unless you lay out the situation, rhetorically, to convince the reader that your different spell-slingers actually carve up magic that differently.
Perhaps the enchanter sneers at a pyromancer lacking breadth of ability. Perhaps a pyromancer sneers at an enchanter for lacking deep understanding and sneering at technique.
There's always the possibility than, as with an ending idea, you will develop it so far that the beginning no longer fits and you must change it.
And there's always the possibility that the beginning peters out, and you can't get a story out of it, at least until you get another idea to add to it.
Such is the writing life.
In My Ending Is My Beginning -- I Hope
You have it in hand, the triumphant ending, the glorious way the hero will defeat the villain and usher in the age of gold where he can marry his love interest in peace.
Inspiration and Aristotle
When dealing with inspiration from scenes, or parts of scenes, trying to develop them into a full story, it is wise to remember Aristotle's dictum.
Yes. Powerful ending.
Spell slinger. Very poetic. Reminiscent of the more colloquial Gun-slinger.