Traditionally the term archangel means an angel of high rank in religions throughout the world, including Christianity, Islam, Judaism, and Zoroastrianism. In the Bible, there is only one named archangel, Michael. The angel Gabriel is considered by most religions to be an archangel, however is never named as anything other than a messenger of God anywhere in the Bible. The books of the apocrypha name two other angels, Raphael and Uriel, not as archangels but as angels performing duties assigned to them by God. In Revelation 9:11it names the "angel of the bottomless pit" as Abaddon or Apollyon, but he is not an archangel, in fact he appears to be an opponent of God. Nowhere in the Holy Bible or anywhere in the books of the apocrypha is there an angel named Cassiel.
It could be that the origin of the belief in the archangel Cassiel may have been from the Kabbalah, which is a mystical aspect of Judaism. Cassiel first appeared as an angel in Judeo-Christian religions after the accepted bible texts were written. Cassiel has been associated with the Seventh Heaven and is sometimes named as one of the seven archangels. In popular culture the character of the archangel Cassiel, with variations of the name, has been a part of the plots of several television shows, movies, books and video games.
There are people that believe in the mystical power of angels and believe Cassiel is an archangel, but that he rarely interferes when calamities or major events happen on our planet, that he is only an observer. Some mystics and ritual magicians even associate Cassiel with the planet Saturn claiming that he is the ruler of Saturn and is responsible for its motion and proper place in our solar system. These practitioners of mysticism believe that Cassiel is an "angel of solitude and tears and presides over the death of kings."
Cassiel is cast as a type of guardian angel by some metaphysical practitioners. Believers in guardian angels say that he is the "angle of peace and harmony" and that if you feel that your life is not balanced and overwhelming you, you should ask for his help. Different believers in angels believe that he is protectorof humans when in a dangerous situations and sell jewelry talismans that supposedly convey the protective spirit of Archangel Cassiel upon them.
Although we have been led to believe that there truly is an archangel by the name of Cassiel by the media and mystics, the fact remains that there isn't a mention of any angel or archangel named Cassiel in holy texts.