The Restless Chirurgeon

Marines on death's door have sometimes been granted a reprieve by the Emperor in the form of aid from a mysterious individual known as "The Restless Chirurgeon". Accounts differ but all describe an apothecary wearing ancient armor (often Mark II or Mark III) coming to their rescue. The Restless Chirurgeon only appears to truly brave souls who are dying alone. Sometimes this entity grants a reprieve to the marine, other times they just stay with them until their passing, and some marines have been offered the Emperor's Peace to them. Many say this is a myth or even some heretical warp entity but many good and honest marines swear they have seen the ancient apothecary at work. It “appears” (around a corner, as a mirage, etc) when it is needed most and just as quickly vanishes after its job has been completed or other brother marines arrive.

The Canticle of the Chirurgeon
Apothecaries pass the tale of The Restless Chirurgeon down from master to apprentice in the “Third Canticle of the Chirurgeon” to explain its presence and extol the virtues an apothecary should strive towards. The canticle recounts that the Restless Chirurgeon was “the first apothecary” (it is unknown if this was a title or if they were literally the first) and they accompanied the Great Crusade. They are said to have watched as their entire chapter was destroyed to the last man during a horrific battle, despite the Chirurgeon’s valiant effort. The last to fall was his own Legion Apothecarion detachment, but they too were killed. In his rage he slew all the enemies, some 12,000 xenos, with this wit and skill before recovering the gene seed of all the fallen legionaries. The hymn recounts that he worked tirelessly for 3 Terran months to recover all the bodies and purge all the xenos and when he had finished he simply moved onto the next battlefield in range of a craft he procured. So he went for the rest of his days, slaying the enemies of the Emperor and recovering the bodies of fallen marines from the most horrific situations possible. So long did he work and so terrible were the wounds he sustained that his armor became scoured and colorless, all markings of allegiance other than the Imperial Eagle and Prime Helix reduced to nothingness. The song ends with first line because the Chirurgeon’s job is never done; he’s still out there saving lives and granting the Emperor’s Peace to those who need it.

Theories
The most common theory is that "The Restless Chirurgeon" is something like the Legion of the Damned and is possibly a being who transcended mortal space and exists partially in the immaterium until it is called upon by a strong enough emotional plea from a marine.

There is probably validity to claims that it’s something more mundane: tall tales, the misidentification of apothecaries, or even a hallucination embedded somewhere in the subconscious of a dying marine (they hope to see an apothecary so they do).

Inquisitors have looked into the matter and some puritanical ones suspect that it is a malicious warp entity giving a coup de grace to marines who would have lived or sucking their souls from their bodies. They take the form of an apothecary because that is what they have found to be least likely to be shot on sight.