Merciless one

Merciless ones are aquatic parasitic aberrations in all likelihood related to faceless ones. They are cycloptic, squid-like creatures with a body shape similar to Ozumat. They attach themselves to the heads of things to enslave them, and can affect their host's mind even when "dead." As the Beak of the Merciless One shows, they have beaks.

Erunak Stonespeaker was abducted by the Naga and enslaved by the Merciless one Mindbender Ghur'sha within the Throne of the Tides. In Twilight Highlands the Merciless one Julak-Doom is controlling a flesh giant.

Known merciless ones

 * Mindbender Ghur'sha, controlling Erunak Stonespeaker in the Throne of the Tides
 * Julak-Doom , controlling a flesh giant in Twilight Highlands
 * Deep Subjugators, summoned from... somewhere during Quest:Final Judgement
 * Merciless Ones, controlling gilblins and working with naga on Deepfin Ridge, at least one was later used as a hat
 * Baby Octopuses, not actual merciless ones... that we know of

Speculation

 * They may be a cousin species to the Faceless Ones, also an ancient aquatic like species who serve the Old Gods.
 * They may also be related to the kraken, Ozumat is called this, but given his ability to attach to victim's heads and control them, he is also thought to be a large variant of Merciless One.

Trivia

 * The merciless ones resemble the Kaled Mutants of the popular British science fiction program, Doctor Who.