"Severus Snape, long-standing Potions master at Hogwarts School of Witchcraft and Wizardry, was today appointed Headmaster in the most important of several staffing changes at the ancient school. ..."
Harry Potter and the Deathly Hallows
I don't understand the phrase "in the most important of several staffing changes". "in the most important of" doesn't look grammatical and I haven't found it defined in any dictionaries. How should we understand it?