The Old One is an ancient Lovecraftian deity and an enemy in the Real Ghostbusters cartoon series.
In order to keep the visiting Ghostbusters from interfering, they stole the book from a Soviet institute, and blamed the Ghostbusters. Meanwhile, Vladimir and Dmitri met up with a cult miles outside of Dnepropetrovsk to summon the Old One from a secret underground temple.
Fortunately, the Ghostbusters made it to the Old One's lair and were able to stop it before it completely made it to the surface. Slimer was sent back home to get "Big Trouble", a huge apparatus that uses the power of all the Proton Packs to create a rip in space-time, like a black hole, to absorb everything it touches. Peter Venkman bravely threw Big Trouble into the pit and ran way as he saw the Old One reveal its hideous form. Then Big Trouble activated and sent the Old One back to where it belonged.
The chant that summons it is in an in-joke : " Brika-Braka-Fyakraka-Cis-Boom-Bah - Old Ones! Old Ones! Rah-Rah-Rah!". This is taken almost line-for-line from a classic Bugs Bunny short, "The Super Rabbit", in which Bugs briefly tricks his pursuers into cheering for him, ala a sports game.