In the Samudra Manthana story, the deadly poison released during the churning was Halahala.
The devas and asuras churned the Ocean of Milk in their search for amrita, the nectar of immortality. Before the desired treasures appeared, a terrifying poison called Halahala emerged. Its fumes threatened all existence, so the gods turned to Shiva for help.
Shiva drank the poison to protect the universe. Parvati is often said to have stopped it from passing into his body by holding his throat, leaving it blue. This is why Shiva is called Neelakantha, meaning blue-throated. Halahala is sometimes spelled Kalakuta or referred to as kalakuta poison in different sources. It must not be confused with amrita: amrita was the sought-after nectar, while Halahala was the dangerous substance that appeared earlier in the cosmic churning.