I created my BBEG with flavor, and now can't come up with a decent build!

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Consider possible prestige classes. Sublime Chord (Complete Arcane) gives him some significant spell casting, but you can't start that until 11th level. Seeker of the Song (Complete Arcane) gives some offensive bardsongs. Virtuoso (Complete Adventurer) has some interesting bardsongs too. Or, and I think I like this one the best, make him into a Beguiler (PHB 2) instead. Beguilers can reasonably impersonate a bard (it's even there in the class description), and have a ton of spells, all based around illusion and enchantment. That would surprise your PCs.