"Those below" term is referred to the natural subterranean inhabitants/creatures1 who lived in the caves under the ground discovered by dwarfs. It is suspected that they are corrupted/ connected to Variphore 15. (explanations below).
The caves were firstly discovered by dwarfes during "promising" mining work of rare and exclusive ore in section 3DA2, however, Gnomes seemed to know already about creatures living below underground.
Upon discovery of the cave system, several dwarfs gone missing 3, and afterwords battles between dwarfs and the discovered creatures started to be regular and more intense. This created a stronger alliance between dwarves and Gnomes4 , and even some of them moved to the hub to provide with resources (crystals and mushrooms) and vital intelligence support to the dwarfs.
"Those below" have several leaders, but 3 of them are recognised as a crucial ones and they are named by dwarfs as: Count of the Core, the Duke of the Depths and the Baron from Below. 5 Ofcourse, this has been added within time and new updates.
"Those Below" are actually the one who flooded tunnels with lava against to defeat dwarfs and gnomes. 6
This is why you will encounter damage from lava when walking around it as also when you enter in Warzone 7-8-9. 7
All is mainly explained in Dangerous Depths Quests 8 , Big Foor Burden Quest 9 and Too hot to handle 10,with additional notes in books "Those Below" 11;-)
Also Some NPC's 12 speaks about them:
Now, In Too Hot too handle quest we are getting to know there was previous a Civilisation 10
Which gives us suspicion that "Those Below" possibly destroyed it.
It is important to mention that part of Opticording Sphere Quest 13 is happening there. As per Dolis 14 Report which we can find in the Hub - they have reports of the monsters from another dimension with connection to Planestrider- this can be connected to "Those Below".
I also suspect that "Planestrider" is part of Variphore Lore.15
Please note in teaser for Too hot to handle it is mention that there are whisper saying:
"You can be one of us. Join us. Join us."
Which is one of the ways of how Variphore "corrupt". This also would explain presence of Gnomevil as a boss.