Intellivore — Diane Duane

The Great Rift lies between the Sagitarius and Orion arms of the galaxy. Stars are scarce there, beyond the authority of the Federation, and legends abound of lost civilizations and of ancient monsters that prey on those who dare to venture into the vast darkness between the stars.
When several ships and colonies mysteriously disappear into the Rift, the U.S.S. Enterprise leads an expedition to investigate various disturbing reports. Accompanied by two other Federation starships, Picard and his fellow captains discover a bizarre menace of unimaginable power. And the only way to trap this destructive entity is to use the Enterprise as bait.