THE CURSE MARK

The Cursed Seal of Heaven is one of the strongest of Orochimaru's cursed seals, the other being its counterpart, the Cursed Seal of Earth. The seal consists of three tomoe (the design known as mitsudomoe), similar to the Sharingan, which, when activated, spread flame-like markings across the user's body. Like all cursed seals, the user receives increased chakra levels and physical capabilities when the seal is active, to the point where with the second level, Sasuke was equal in power to Naruto's one-tailed form. The seal was applied at the left side of the back of both Sasuke's and Anko's necks.
Anko received her cursed seal when she was a student of Orochimaru. She, along with nine others, were the first people to receive a cursed seal. She was the only one to survive the application, establishing the one-in-ten success rate for the procedure. Because she lacked the will to use it, her seal receded, and Orochimaru wiped her memory. Orochimaru was able to briefly reactivate the seal when they fought in the Forest of Death before the Third Hokage sealed it off again. It was fully removed when Sasuke used it as a medium to revive Orochimaru's consciousness and body via the Evil Releasing Method.

Orochimaru branded Sasuke with the cursed seal in the second phase of the Chūnin Exams, which took place in the Forest of Death. Orochimaru intended to have the seal corrode Sasuke's mind so that he could become his next body. Kakashi Hatake used the Evil Sealing Method to try and prevent this, but its effectiveness relied on Sasuke's choosing not to use his cursed seal. Sasuke ultimately chose to advance his seal to its second level, and went to Orochimaru with the help of the Sound Four.


Sasuke's second level of the cursed seal.
When Sasuke activated the second level of the seal, his skin turned dark-grey and his hair grew and turned dark blue. His eyes also turned dark grey. Additionally, he grew webbed-claw-shaped wings from his back which he could use to fly and glide, and a dark, star-shaped mark appeared across the bridge of his nose; during his battle against Deidara in Part II, was able to replace a lost limb, like his left wing that was destroyed from one of his explosions, and replaced it with snakes. During Sasuke's battle with Itachi Uchiha, he lost the cursed seal, along with Orochimaru (having earlier been absorbed into Sasuke's body), as they were removed with the Sword of Totsuka.
According to Kabuto, the seal itself contains Orochimaru's chakra, which Kabuto used in order to improve the binding power of Summoning: Impure World Reincarnation. In fact, when Orochimaru was defeated, his consciousness remained split amongst the cursed seals, and Sasuke used that, along with a piece of Kabuto's modified flesh, to bring the former back to his original state and mind