The term 'POISONOUS POTION' can be summarized by the crossword solution of 'HEMLOCK', which is a seven-letter word. HEMLOCK is a highly toxic plant that has been used as a poison since ancient times. It was famously used to execute Socrates in 399 BC. The plant contains a number of alkaloids, including coniine, which is a neurotoxin that can cause paralysis and death. Ingesting even small amounts of HEMLOCK can be fatal, making it a dangerous and poisonous potion.