A wormhole leading to a galaxy billions of light years away blinks into Earth's orbit every 3 months and can only be seen from a hill with stones placed by ancient cultists.  These cultists would communicate with the creature on the other side.

 

Rlim-Orth has remained on his side of the wormhole for centuries, having long lost interest in the world on the other side since the cultists, who once worshiped him, have gone extinct.

 

The creature is about 80,000 miles long and is wrapped several times around a barren planet.

 

 

Image from Tales of Monsters - Cosmic Abyss.

Rlim-Orth passes through the wormhole.

 

Image from Tales of Monsters - Cosmic Abyss.

The giant worm consumes a victim.

 

Image from Tales of Monsters - Cosmic Abyss.

The giant worm consumes a victim.