Mee-Krah was an immensely powerful entity that awoke every thousand years to consume ghosts and replenish its ectoplasmic energy. As it fed on more ghosts, its temperature rose and it scorched the earth in its wake, creating deserts.
As mentioned Mee-Krah awoke every thousand years, feeding and returning to a dormant state. Egon stated it had created the Gobi, Sahara and Death Valley, indicating it is at least 3,000 years old, if not much older. It was so powerful and feared, that other ghosts flocked to the Ghostbusters' Firehouse to be captured rather than face it and to be devoured by the behemoth creature. However, this only led Mee-Krah to target the Containment Unit. Initially, the Ghostbusters tried to lure it out to sea but the ghosts used as bait objected and messed up the plan by hiding in Ecto-2.
At the old Hudson River Pier across the New Jersey shore, the Ghostbusters staged a second confrontation with Mee-Krah. The combined firepower of four Proton Packs and Ecto-1's Proton Cannon had no effect on the entity. It was only neutralized by a device invented by Peter Venkman called the Ghost Attractor. It turned out to be the world's first Ectoplasmic Energy Capacitor. The Capacitor collected, stored and subsequently fired a very large surge of energy. from 1,612 ghosts it had been in close proximity to, at Mee-Krah. Upon making contact with the energy, Mee-Krah was vaporized.
Mee-Krah physically resembled a squid or a jellyfish, possessing a translucent mantle like defensive shield and a number of tentacles. Inside this mantle was a head, containing one eye and a large maw, capable in drawing in ghosts. Despite possessing numerous appendages, Mee-Krah apparently had little use for them, instead burning or firing powerful energy discharges at/through whatever obstacle or opposition it encountered.
The Real Ghostbusters