You need three items,
1) Moonblade, you get this after watching a cut scene inside one of the buildings where an elf sacrifices himself in order to slay a Demon.
2) Symbol of Ri~~, you get this by solving a puzzle that guards a chest inside one of the buildings.
3) Cup of Life, kill the Black Dragon and retrieve this item from it's body.
Next, place all items in the container in the temple.
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If not a Sorcerer, then a Paladin.
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Baldur's Gate Item Mart