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Core items are Athene's Unholy Grail, Rabadon's Deathcap, Zhonya's Hourglass, and Sorcerer's Boots.
Anything you add is situational at best. Morellonomicon isn't advisable because you'll hit max CDR with just Grail, blue buff, masteries and/or runes. You should instead work on adding more AP/MPen, and HP (or toughness in general).