Gozer the Gozerian
Gozer is an ancient, and all-powerful Sumerian shape-shifting "God Of Destruction". It was allowed to enter our world via a supernaturally-charged rooftop temple constructed by Ivo Shandor, in the early 20th Century for this express purpose. The temple can only be opened by the joining of Gozer's two loyal minions, Vinz Clortho and Zuul - The "Keymaster" and "Gatekeeper" respectively.
According to Vinz Clortho Gozer had a "colorful" history:
"Gozer the Traveler. He will come in one of the pre-chosen forms. During the rectification of the Vuldrini, the traveler came as a large and moving Torg! Then, during the third reconciliation of the last of the McKetrick supplicants, they chose a new form for him: that of a giant Slor! Many Shuvs and Zuuls knew what it was to be roasted in the depths of the Slor that day, I can tell you!"
It was only allowed to change forms one time after crossing into our world - The Stay Puft Marshmallow Man, as chosen by a reluctant Ray Stantz. Gozer was banished back to it's own dimension by the Ghostbusters combing the power of their packs, called 'Crossing the Streams. The Gozerian Temple was destroyed, thus sealing the gateway.