At first it was just a name, picked by Feiht when she summoned Leaf a magical staff to get past some troublesome guards. Later there turned out to be some power behind the name, as mentioning it summons thunder. Soon after it is revealed the Magister Malvenicus is a real person, an old human mage who has mostly retreated from the world and who may, in fact, be slightly insane.
Malvenicus used to be more cavalier about using his powers and the thunder-name was a spell he cast when he was young and it seemed like a good idea. However he seems to have changed over the years and by the time Leaf and the Gang (minus Feiht) run into him he eschews most power and has gotten rather bored and annoyed by the frequent thunder.
Malvenicus has a huge floating castle that turns into a snowglobe while not in use, and which he stores in his satchel. The castle was originally home to several families of wizards, but now only Malvenicus lives there. It was created several centuries ago and since then many powerful wizards have added their own extensions to the point where the whole thing is a mess of magic and illogic, with passages twisting in all dimensions. A comprehensive map of the castle would make a pixie dizzy. Amongst other things, the many-towered castle contains a magical maze, an eternal fire room, an eternal snowstorm, a dragon's lair (though this was not by design) and, for reasons that may best remain unknown, a kids alligator pool.
Malvenicus sees himself as a bit of a guardian of the local people, even though not all appreciate his efforts. He walks the land healing, teaching and hopelessly confusing people. His long years have made him rather weary of power and if he feels somebody will abuse power, he will use his magic to stop people from obtaining it. He refuses to teach magic, but will nonetheless test aspiring mages to make sure they can be trusted.
Reckless in his youth, Malvenicus used his magic in abandon, often with little regard for the pains he was unintentionally inflicting on others. This only ceased when a pixie stole his staff and his subsequent weakness forced him into closer contact with 'ordinary' people.
While he originally meant to replace his staff, in the end he never did, feeling that he was not the best guardian of such power. Instead he went into hermitage, traveling the lands in the guise of an old conjurer. Over time he regained much of his magical prowess even without a staff, but he hesitates to use it.
When Leaf returns his staff, he immediately senses that the elven boy has magical powers, but he also senses the furies and decides to put Leaf's conscience to the test. Despite continuing misgivings about the furies he accepts that Leaf controls them for now, but warns that they cannot do anything but destroy things. He then gives Leaf his staff so that the boy would have magic that could more easily do good, although his true reason is that even now he does not trust himself to wield such power.