Dolreans

The large militant race of the Shackleton Expanse. Never been seen in person due to their habit of not leaving survivors (only taking them as slaves), and using self-destructing individual crafts in conflicts.

History
The Dolreans have been dominating the races of the Shackleton Expanse for decades. The Kierans have been defending themselves by maintaining a low profile and scanning all unknown vessels psychically before letting any ship know where they live.

They practice slavery and use other tactics (such as suicide bombers) that are directly opposed to Starfleet ideals.

They take slaves and had enslaved a race called the Calia. The Calia have recently been liberated by the Kierans.

Hive Ship
A large-scale invasion/raiding vessel, much larger than most standard starfleet vessels, on a scale with a Borg Cube.

Suicide Bombers
Small, one-person ships made to be launched and flown directly into the hull of an enemy ship. Once they make contact the payload is detonated, killing the pilot in the process. A Hive Ship will carry several dozen of these smaller craft.

Standard Vessel
Smaller, mid-range ships meant to be used as recon and security. These also will hold a couple shuttle-size crafts inside.

USS Nightingale
The Dolreans were first heard of by Starfleet when the USS Nightingale was told of their actions by the Kierans.

The first physical interaction was also with the USS Nightingale, an engagement during the Omega Raid with two of their Standard Vessels. The Nightingale was able to take both out, but was left crippled and took many days to recover.

The Nightingale (and it's shuttle, the USS Astartes, independently), pick up an SOS on the way back from the Omega Raid. Upon arriving on the scene the Nightingale discovers the USS Bellerophon has been completely neutralized. Studying the wreckage and active suicide ship embedded in the Bellerophon's hull, it becomes clear that an entire Hive Ship was taken out.