What is the common name of the poisonous plant Helleborus niger?

The common name of the poisonous plant Helleborus niger is Christmas rose.

Helleborus niger is an evergreen perennial in the buttercup family, Ranunculaceae, not the rose family. It produces large white flowers in winter or early spring, which inspired the seasonal common name. A Christian legend says the flower sprang from the tears of a girl who had no gift for the Christ child.

The plant is poisonous because different tissues contain several defensive compounds. Damaged leaves and stems can release protoanemonin from ranunculin-related glycosides, causing irritation and blistering. The roots contain cardiac-active compounds such as hellebrin and related substances, so ingestion can affect the heart as well as the digestive system.

“Lenten rose” is a common name for other hellebores, especially Helleborus orientalis, and “winter aconite” and “snowdrop” are unrelated winter-flowering plants. The name “black hellebore” is also used for H. niger, probably referring to its dark roots rather than its white flowers.