Summary
The Belisarian Furnace is an auxiliary organ included in Cawl’s Mark X power armour pattern for Primaris Astartes. It functions as a stimulant-and-repair cluster: a one-shot organ, internal to the armour, that releases a flood of metabolic and combat-stimulants and accelerates biological repair. It can fire only once between extended rest periods.
Status as relic
Strictly speaking the Furnace is not a relic in the archival sense — it is contemporary archeotech, designed in modern times by Cawl. But the design draws on multiple recovered STC fragments and on Cawl’s millennia of accumulated study. The Cult Mechanicus accepts it as recovered-and-reconstructed, which places it in a borderland between relic and new manufacture.
Doctrinal note
That a single living priest could design a working organ from STC fragments and accumulated study is, for Cawl’s critics, evidence that he has overstepped the proper bounds of the Quest for Knowledge. For Cawl’s supporters, it is the Quest in action.