"It's a plant toxin from South America. Tribes there once used it to poison dart tips, and for a while doctors used it to keep people still during surgery. But now it's considered too dangerous. There's no reason for Lester Liu to have it. We found it sitting on the counter in the kitchen. We replaced it with tap water."
Tubocurarine is a neuromuscular-blocking drug or skeletal muscle relaxant used adjunctively in anesthisia to provide relaxation during surgery or mechanical ventilation.
Lester Liu tried using it to keep Oona still during the Empress Awakens.