What it does
I'm not familiar with "Polymorphine" as an official Magic: The Gathering keyword or ability. It doesn't appear in the comprehensive rules or any card set I can verify, so I'd be guessing—and giving you confident-sounding but fabricated rules text would do more harm than good at your game table.
A few possibilities: you might be thinking of a similar-sounding mechanic, a custom/unofficial card, or a misremembered name. Some adjacent real things include "polymorph" effects (like the card Polymorph, which sacrifices a creature and digs to put a new one onto the battlefield), shapeshifter/Changeling cards, or transform/disguise mechanics.
If you can point me to the specific card or set where you saw "Polymorphine," or paste its reminder text, I'll gladly break down exactly how it works and where it shines in Commander.
Iconic cards with Polymorphine
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