The Crimson Stone

Crimson Stone
The Crimson Stone is one of two of the greatest treasures to the vampires, the other being the Ebony Stone. Both were believed to have been lost. The Crimson Stone uses the soul of a vampire to give its user power; however, at the same time, it also bears the vampire's curse, meaning that whomever uses the Crimson Stone will turn into a vampire. This was the case for Mathias, as he was in the midst of the creation of the Crimson Stone.

History
The Crimson Stone was created by Mathias Cronqvist. His scheme was to use the Crimson Stone to obtain eternal life and defy God for the death of Elisabetha. When Leon defeated Walter, Death immediately took Walter's soul to use for the Crimson Stone. The Crimson Stone became complete, granting Mathias eternal life and the curse of the vampire. He became the Lord of Darkness and the Crimson Stone has remained in his posession ever since.

In Dracula's Posession
The Crimson Stone holds the soul of the powerful vampire, Walter Bernhard. With the soul of a powerful vampire, Dracula was granted his incredible powers. Since the Crimson Stone is used for capturing vampire souls for use as power, the Crimson Stone, in a sense, grants Dracula the gift of resurrection. Whenever Dracula dies, his soul is released and eventually goes to the Crimson Stone, giving him his life back. This endless loop is one of the reasons Dracula constantly revives, the other reason being that someone intentionally resurrected the Dark Lord.

In a lot of the Castlevania games, the player is required to hit the pendant around the Count's neck or his head. Although only speculation, the reasoning behind this might be because the Dark Lord cannot be killed without the Crimson Stone being damaged. By damaging Dracula's power source, it may weaken him and result in his death.

Due to the fact that Dracula was destroyed in 1999 by Julius Belmont, it is possible that Julius somehow managed to destroy the Crimson Stone, ending the infinite resurrection cycle and preventing Dracula from being brought back to life in the future.

Trivia

 * The Crimson Stone appears around Dracula's neck in every game, with the exception of Haunted Castle, Castlevania 2: Simon's Quest, Belmont's Revenge, Castlevania: Bloodlines, Castlevania 64, Harmony of Dissonance, and Dawn of Sorrow (Although it isn't the true Dracula in DoS.)
 * The only known vampire's soul contained within the Crimson Stone is that of Walter Bernhard.
 * It has not been in the posession of any other vampire other than Dracula.
 * Dracula has command over Death because of the Crimson Stone.