for one its called the solarium, and second every time you change the phase of the moon you need to defeat the groups of enemies that becomes available when that astral bridge appears for each of them upon defeat of all of these and the astral bridge to the next boss in that area opens proceed to him, but i must warn you he has his own set of rules for one don't cast the same school spell on him twice or he will hit you with a ice colossus ( or was it helephant?) every time you hit him he will place a tower ward but if you cast a blade on yourself he will remove his own tower ward saying " you have charmed your way into my heart"
