Making magic seem real is a difficult task. Casting a fireball or lightning bolt is a common skill for powerful wizards, which is why I never use those spells. First, they have been used before, so there's nothing new or novel about them. Second, those spells are clearly documented in several Dungeons & Dragons handbooks, so either you must carefully follow the rules or risk losing interest in your book by the gamer community, many of whom are avid readers.

     Were I to use fireballs or lightning spells, then I would make them special or unique ... and definitely dangerous. Perhaps I would have them severely burn the caster's hands, requiring a healing spell if they aren't wearing thick leather gloves, or burn the required wand to a crisp. Fireballs explode; cast at close range, the effects of a nearby fiery explosion could seriously harm the caster. Lightning can't be easily directed without a wire or a stream of water to vaporize; just because you cast a bolt of lightning doesn't mean that it can't arc back.

     Magic isn't interesting in what it can do; magic is interesting in what it can't do. In every story, a reader has problems; if they have a magic ring that can do anything, then all of their problems can be solved and the book is over. The more powerful the magic is, the less interesting the magic is.

     Magic (in stories) has three common limitations: temporariness (magic that quickly wears off), specialization (magic that only does one thing), or expensiveness (the cost to cast the spell is great). Expensiveness can be rare materials or a debilitating weakness; in most sword & sorcery, magic weakens the spell caster slightly, but they always have just enough strength to cast one more critical spell. Sadly, the 'one last spell' plot device has been used so many times that it is predictable and often disappointing.

     Many books have been written about magic, from Wicca to Satanism. Reading them can help add a unique flavor and style which is missing in many novels. Many ancient civilizations have different practices of magic, from the bottle of the genie to the blood-sacrifices of the Aztecs. Try to add something new to your magic; if the required component is a precious gem, then the price for each spell is high, and when the gems run out, then there is no more magic. If the price is the magician's blood, then too high a price can kill the magician.

     The Seer is a unique magician; even when he tries to cast a simple spell, he attempts to justify what he thinks happens by the best science known in the Dark Ages, which has all been discredited (but not until centuries later). His speculations add color to his spellcasting, and he tries to use his spells in ways that are special, not just 'cast the spell and solve the problem'. His spells are also limited; he can't just make anything happen, and even something as simple as a headache can prevent his spells from working.

     If the Seer could do anything, then his story would never have involved the companions; he never would have needed their help. It is his limitations, the fact that he is still learning magic and hates his limitations, which makes him interesting.

     Mostly, magic has to be special to your story, not stolen from some other fantasy novel or video game. Magic should be a system, a faith, or a mythology; something that is more than just a plot device. Look at Hogwarts; Harry Potter hosts a system of magic and learning magic that can be described without mentioning any of its students or teachers. Systems of magic can be as unique as any character, and they add just as much detail to your story.