Basically, it allows Alarielle to re-cast the same spell two times in a row. If I were to take Life magic with her and cast Flesh to Stone once successfully on a unit (say Dragon Princes), increasing their toughness by +2, could I then use this item and recast Flesh to Stone on them again to increase their toughness by +4? (And if Throne of Vines is active, potentially increase it to 10 [Maxing out at 10 I'd assume]?)
My armies: Empire, Dwarfs, High Elves, Wood Elves, Dark Elves, Lizardmen, Skaven, Tomb Kings, Ogre Kingdoms, Orcs and Goblins, and Skitarii (40K).
You're correct, however the true strength of the item is to allow her to cast a spell if she has failed in the same. It can be devastating when your counting on casting a spell at a critical time only to fail and then the Wizard is done for the phase. This gives the Everqueen another chance, and so is a great failsafe item.