The vast majority of red-aligned Phyrexians dwell in the so-called Furnace Layer, a relatively new, interstitial layer between Mirrodin's outer surface and inner core. This layer's creation was one of the first steps in transforming Mirrodin into New Phyrexia, built from the ancestral memory embedded in the glistening oil. Because industry is the primary purpose in the Furnace Layer, Phyrexians there have been adapted from their prototypical forms to more specialized functions.
Access to all five of Mirrodin's mana-rich suns has led to diversification and disunity, damaging the Phyrexian singularity of purpose. Of all the energies Mirrodin introduced to the Phyrexian ethos, the mana from the red sun has been most challenging, because it's the force that lies behind the concepts of individualism, compassion, emotion, and freedom.
The mana from the red sun gave rise to Phyrexians who had just a glimmer of concern for other lifeforms—not full-blown compassion, but enough empathy to cause hesitation, a phenomenon more or less alien to Phyrexia. Beings of varying levels of sentience reacted to this impulse differently. Among nonsentient creatures, this primitive empathy simply caused moments of confusion before action, but fully sentient Phyrexians found themselves deeply conflicted and even ashamed of their concern for others. Make no mistake—Phyrexia, even when influenced by the red sun, is still a brutal and horrific system. Most Furnace Layer denizens do what they were created to do: tend the molten slag and turn the scraps of Mirrodin into the hellish landscape of New Phyrexia. But the influence of red mana has caused this part of New Phyrexia not to fall in lockstep with the other factions.
Faced with impulses and hesitations they didn't know how to cope with, the Phyrexians of the Furnace Layer cleaved tightly to their function: to keep the fires of the furnaces burning, to incinerate dead and failed organisms, to reprocess the metals of former Mirrans, and to turn those metals into the raw materials for new Phyrexians and new nested layers of the plane. This adherence to labor afforded the conflicted Phyrexians a measure of resolve in the face of their sublimated dissent against the broader Phyrexian disregard for individual beings.
For the least sentient of them, apathy toward other beings became the norm. For the most sentient, industry became a means of demonstrating to other Phyrexians that they were part of the system, that they were functioning correctly, that they had a use, despite their secret doubts.
The furnace denizens cooperated with the Phyrexian invasion of Mirrodin's surface world; they aided exactly as much as asked and no more. When the Mirrans on the surface found themselves outnumbered and outgunned, a few did the only thing they could do to survive: they retreated into the lacunae, especially through Kuldotha and Taj-Nar, but also at Lumengrid and the Araneas Altar. Only Ish-Sah was blocked off completely to Mirrans.
In the initial days of the appearance of Mirrans at the lacunae, the furnace Phyrexians killed and incinerated them. But as the days passed and more and more refugees appeared, a hesitation grew. The sentient beings appealed to their praetor, Urabrask, for guidance. Urabrask took days to reply. When he did, his three-word decree stunned the others: "Let them be." But none dared challenge his decision, and most tacitly agreed, although their subordinate nature prevented them from voicing that agreement.