"The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips." Firenze the Centaur
The blood of a unicorn, not regularly used in potions
Unicorn blood is silver-blue in color and shines under moonlight (PS15)
To allow himself to survive in Quirrell's body, Voldemort induced Quirrell to kill unicorns and drink their blood. Unicorn blood will give the drinker only a cursed life because the drinker has killed something defenseless and pure has been killed only to save his or her own life. Voldemort didn't care, of course, because he was only trying to survive until he could drink the Elixir of Life(PS15).