"Thousand Years of Death" is a secret art in Naruto known as the Shadow Clone Jutsu. It involves thrusting a tiger seal towards the enemy's anus, causing severe damage. Kakashi Hatake created it but only used it once. Naruto Uzumaki learned and improved the technique, using it against Gaara.
Kakashi's ability to use the Eight Inner Gates is not surprising because he possesses the Sharingan. Although his friend Maito Gai is the master of the Eight Inner Gates, Kakashi can only open up to the second gate. He used it once to increase his strength when his stamina was low. Kakashi's strongest technique is Kamui Raikiri.
After obtaining Obito Uchiha's Mangekyou Sharingan, he gained this move. Kamui Raikiri is surrounded by black lightning which sends the target to Kamui's dimension upon impact. Kakashi used it before losing Obito's eyes. Although the Multiple Shadow Clone Jutsu is Naruto's exclusive technique, Kakashi can use it as well.
Due to his limited chakra reserves, he rarely utilizes it. Kakashi used it once to intimidate civilians but didn't display its true power. The Rasengan was created by Minato Namikaze, the Fourth Hokage, and Kakashi learned how to use it since Minato was his teacher. He used it once while instructing Naruto on chakra nature transformation. However, the Rasengan is not suitable for Kakashi's lightning nature chakra.