History and specifics
It was the smallest black hole in the space belonging to the Cardassian Union and was believed to have been one of the oldest, which led to some mistakenly believing that it was an imploded star. The Eye dated from the formation of the universe according to the legends of the Cardassians, though cosmologists tended to agree with them. The Cardassians discovered it with telescopes long before they developed space travel. They did not know what it was at the time, and later named it after the mythological monster Talek.
Typical black holes kept the same mass they had when they were stars, but ones like the Eye of Talek had an unknown origin (though it was believed such small ones were ancient). In appearance, it resembled a hole that punctured the fabric of space and was notable for an absence of stars and a golden halo of gas as well as dust that was streaming into it. This phenomena was the size of a saucer section of a large starship but was completely black in color.
In 2373, the Dominion dispatched a mining vessel to harvest the corzanium in the mouth of the Eye of Talek, as the substance was required for the artificial wormhole they were attempting to create which would link the Alpha Quadrant to their strongholds in the Gamma Quadrant. (TNG - The Dominion War novel: Behind Enemy Lines)