The so-called "Horrors of Halo Device" refers to those who were once human, but transformed into sinister xeno monsters through the use of strictly prohibited Xeno Artefact "Halo Device". they have a humanoid shape but obviously no longer human, but some kind of Xeno monster that is far more inhuman than the Eldar and Tau.
https://wh40k.lexicanum.com/wiki/Halo_Devices
Their common morphological descriptions:
“remaining humanoid but no longer human. Instances of burning, radiant eyes, corpse-like flesh, distorted gaunt features, mottled or armoured hide-like skin, and even a fine beetle-like carapace are all recorded, and almost invariably the fingers of the hands transform into long talons of exposed bone”
The miniature conversion is based on this description above, but there's dont need to be too restrained as no two Horrors are alike.
puts simple:
1.It can be used in kill team game or full-scale war game. usually it's scene such as "a Inquisitor and its henchmen fight against a Horror and its minions".
2.It uses a 35mm or 40mm base. It has a barely human shape, uses human armor and weapons but is clearly an xeno monster.
The key is their head bits. the Vampire heads from the WFB/
AOS are the obvious choice, Tau, Kroot, and Vespid heads are also suitable but need some modification. Maybe there are some more novel choices?
3.it has similar ragtag dedicated units as the Inquisitor henchmen. they are usually servitors because no sane men willing to take orders from a man-eating xeno monster, and if they are crazy enough to obey or are forced into battle, they are more likely to defect or flee in the middle of battle.they servitors minions usually normal armed servitors and Ogyrn servitors imported from the Necromunda game.If they have any human underlings, they are usually renegade maruders from the
IA13, but with some serious leadership penalties since they will have to get close enough to clearly know that their employer is a xeno monster.