Flask of Pure Death

Currently, the only way to learn this flask is to "Discover" it. In TBC, alchemists discover the new flasks by making already known potions.

Old flask recipes can be got by doing rep grinds;
Flask of Chromatic Resistance: Exalted - Lower City
Flask of Distilled Wisdom: Exalted - Cenarion Expedition
Flask of Supreme power: Exalted - Keepers of Time
Flask of the Titans: Exalted - The Sha'tar

Interestingly, There is no known new, upgraded Flask of supreme power.

Requirements to make a discovery

You must be crafting a potion, elixir, flask or performing a transmutation that was added in TBC.

As a rule of thumb that's anything that is learnt from a master alchemy trainer, dropped from a mob in outland or purchased from a vendor in outland. The only real exceptions are the pre-TBC flasks that have been made available from faction vendors in outland.

NOT requirements to make a discovery

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• You don't need to be in outland
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• You don't need a mastery
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• You don't need to be 375 alchemy
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• You don't need to be level 70
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• You don't need to craft yellow or orange recipes to make discoveries. Even grey recipes can proc discoveries
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• You don't need to be crafting a related recipe *except* in the case of cauldrons (You can discover flasks when making potions for example)
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List of recipes learned by discovery (accurate as of 29/01/2008)

Flasks

• Flask of Blinding Light
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• Flask of Fortification
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• Flask of Mighty Restoration
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• Flask of Pure Death
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• Flask of Relentless Assault
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Potions

• Super Rejuvenation Potion
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Transmutes

• Earth to Life
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• Life to Earth
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• Shadow to Water
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• Water to Shadow
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• Fire to Mana
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• Mana to Fire
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Cauldrons - To discover these you need to be crafting the related major protection potion. Cauldron of Major Holy Protection is believed to be non-existent. The discovery of these recipes is much more likely.

• Cauldron of Major Arcane Protection
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• Cauldron of Major Fire Protection
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• Cauldron of Major Frost Protection
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• Cauldron of Major Nature Protection
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• Cauldron of Major Shadow Protection

I just discovered this while making a cata flask Flask of the Draconic Mind. So they can still be discovered with new recipes.