OK prepare for a hell of a fight
Make sure you have a Rogue Stone, and go to the Bridge District. There is a door near the water where you can only enter with a Roguestone. The Staff of the Magi can be obtained inside.
As for the Robe of Vecna, you'll need to download the bonuscd files, try
www.bgdungeon.com/bg2/ When you've installed the files, there will be a merchant called Deidre in the Adventurer's Market and one called Joluv in the Copper Coronet. Deidre has the Robe (bring lots of cash
)
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Melusine, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia