Regeneration potion

"- it is an old piece of dark magic, the potion that revived me tonight -"

- Lord Voldemort's description of the Regeneration Potion

The Regeneration Potion is a dark potion that restore those who have bodies destroyed but still live on due to having a horcrux to a physical body. The potion requires a cauldron large enough for a grown man to sit in as well as three crucial ingredients: bone from the dark wizard's father, flesh from one of his servants, and the blood of one of his foes. The remainder of the ingredients, if any besides water are needed, remain unknown.

It is likely that the potion either required certain rare and exotic ingredients or was a near-forgotten part of magical practice that was not uncovered until sometime after his servant returned to him otherwise Voldemort would likely have attempted to use it earlier.

Voldemort's rebirth
"The Dark Lord shall rise again!"

- Wormtail as he completed the potion to restore his Master in 1995.

In 1994 Lord Voldemort had Wormtail prepare the potion, save it's most crucial ingredients, for his rebirth. While Voldemort had the flesh at hand due to Wormtail's presence and could easily gain his father's bone from his grave, he wanted Harry Potter's blood to rise "greater and more terrible" then when he fell. Through a complex plan one of Voldemort's loyal servants managed to send Harry to him. With the three ingredients Voldemort was able to regain his new body.

The use of Harry's blood in the potion would be a mistake as it prevented Voldemort from killing Harry as the blood passed Harry's mother's protection to Voldemort and anchored Harry to the living world through Voldemort, ensuring his defeat three years later.