Varenach was a creature of the planet Garid which was fed on energy and was capable of phase-shifting. When the Followers of the Lawgiver Avatrunei left the planet for Frigis, they took some species of their planet with them, including some varenachs. However, the Varenach was eventually exterminated on Garid.
One varenach, or a related species, was given by the Garidians of Frigis to the Ferengi trader Aramut in exchange of some modern technology. Aramut sold the varenach to the Morassian Preserve which Iydia arranged to tag it as a sultis.
This varenach managed to escape and not only killed other creatures by draining their neural energy, but also destroyed containment field generators and caused energy shortages in the installations. Eventually an away team from the USS Enterprise (NCC-1701-D) aided by Dr. Vi Hyunh-Foertsch trapped it by activating an harmonic collector in the shelter, capable of emitting high-energy EM fields to lure it. Once the varenach was there, a containment field trapped the creature.
the Garidian T'bak noticed the similarity of the creature to the legendary Varenach of Garid and speculated that it would have survived only by the mans of the Followers, and tracing its origins would eventually lead to their lost colony. (TNG video game: A Final Unity)