Zuul

Not much is known about the Zuul. Even their name is based off of a Lormean myth where the Zuul are described as demons made from shadow as part of the war between heaven and hell.
In 2377 the USS Nightingale encountered a derelict in deep space that had several Zuul on board. The Zuul seemed primitive and feral mindlessly attacking the crew of the Nightingale. Autoposies showed that their brains were highly sensitive to psychic energies which explains why they are drawn towards psychic crew like Lt. Sel. Their fur was thick and protective and has a nanocarbon structure that traps light giving them their pitch black appearance and making them highly resistant to phaser fire.
It is believed that there is another species, one that is highly psychic, that controls the Zuul and directs their actions but so far this "psychic master" has not been discovered yet.
In terms of technology it isn't believed that the Zuul are capable of building the technology that they use and might be similar to the Pakleds where they are a scavenger species cobbling together a variety of alien technology to create functional interstellar craft.
Still despite their primitive behavior it is important to not forget that the Zuul seem to be behind the destruction at Seku VI.