When a person drinks the Draught of Living Death, they go into a deep sleep so strong that they appear to be dead, hence the name. It is made from powdered root of asphodel which is added to an infusion of wormwood, and was first mentioned in Harry's first potions lesson with Snape in Harry Potter and the Philosophers Stone.
It was later mentioned in his first Potions lesson with Horace Slughorn in Half-Blood Prince when Harry receives top marks in Horace Slughorn's first NEWT (Nastily Exhausting Wizarding Tests) Potions class for brewing this potion so well, having used the handwritten advice in the Half-Blood Prince's book. He is then awarded a small bottle of the potion Felix Felicis as a prize.
This potion is described as changing in colour during the process of being made. It has a texture and consistency akin to that of clear water. A process of examining the potency of the potion is to drop a new or fresh leaf in it – the leaf is instantaneously reduced to ashes.