Hond

The  were "the primordial ooze" given sentience as an embodiment of the concept of pain. Hailing from the Dark Times, they were hypothesised by the Tenth Doctor to have been the result of "the Universe not quite knowing what life was like yet" when it created them. Thanks to a time paradox, the Hond eventually emerged into a later era of the universe and threatened the Dalek Empire, forcing the Dalek Prime Strategist to seek out the Doctor for help.

Biology
The Hond, as the sentient embodiment of a concept, could only be destroyed if freed of the pain that formed the core of their being. Individual Hond, being less full of suffering, were capable of some amount of reason, but proximity to the rest of the Horde would make them go more and more incoherent. The horde was capable of travelling through space in the loose form of a sphere. (COMIC: Defender of the Daleks)

Like most lifeforms from the Dark Times, (PROSE: The Knight, The Fool and The Dead) they were immune to death and, indeed, capable of a form of regeneration. When blasted by a Dalek gunstick-fire, a Hond would collapse into ooze for a few moments, but actually feed on the suffering the blast had caused it and reform stronger than before. (COMIC: Defender of the Daleks)

History
After waking up to find Daleks at the doorstep of his TARDIS, with the Last Great Time War seemingly having never happened, the Tenth Doctor was recruited by the Golden Emperor to help defend Skaro from the Hond. The Hond had returned and were seeking to destroy the Daleks, before, according to the Emperor, destroying the rest of the universe and then themselves.

On Skaro, the Doctor and the Dalek Prime Strategist entered the Vault of Obscenities where they confronted their first Hond, who declared that "soon the Hond will kill all." After dispatching him by trapping him in a crystal cage, the Doctor got to work on the matter of defeating all the Hond.

Eventually figuring out their true nature as creatures of suffering, the Doctor managed to rewire all the weapons in the Vault into a huge psychic transmitter, hitting the Hond horde with a ray of magnified relief. As they began to evaporate due to the hurt being gone, one Hond thanked the Doctor for this act of kindness. (COMIC: Defender of the Daleks)

Behind the scenes

 * As "the primordial ooze given sentience" and becoming a time-travelling enemy, the Hond are reminiscent of the ancestor cells from the BBC Eighth Doctor Adventures, a potential identity of the Enemy in the War in Heaven, although a formal link between the two is not drawn in COMIC: Defender of the Daleks.
 * It should be noted that although it could only be normal Dalek parlance rather than a reference to "the Enemy" with a capital E, the Daleks in Defender of the Daleks at one point shout that, "THE ENEMY IS COMING! THE ENEMY IS COMING!", when the Hond are approaching Skaro.