Lore Theory: Custodian Stalkers are left-behind Old Ones.
I have a crazed theory that the Custodian Stalkers are the degenerated remains of the Old Ones in Varlamore, and it's been bothering me for a while. The design and concept art of the Stalkers, their ties to the Strangler and their compulsive sacrifice of others to it, and lastly their auspicious *lack* of a history. Let me present some evidence so I look less crazy.
During the Garden of Death quest, the Old Ones' tablet reads: "Many body needed. Body need life. Feed body soul vessel.-" and "Transfer method success. Body success. Soul vessel success. Time arrived. Death poison needed. Death create life.-". The Old Ones needed many bodies to transfer their souls into, and 'made' the 'body'. The 'Body' that the Old Ones were to use was likely crafted or mutated with alchemic herblore, of which the Old Ones were notable masters. These bodies were then fed the 'Soul Vessel' that would house the Old Ones' souls; some component piece of what would become the Strangler. The Strangler didn't transfer or house the soul properly though: "Plant create dark death. Send home. Home destroyed. No life. Soul vessel fail-". But it never says what happened to the 'Bodies', only that they didn't transfer the Soul. So, theoretically, these 'Soulless Bodies' of the Old Ones were still around, infected by the modified Strangler and supposedly immortal.
Concept Art of Custodian Stalkers
The concept art of the Old Ones and the concept art of the Custodian Stalkers matches up very closely when put side by side. Both retain the satyr-like ante-dactyl legs and antlers, as well as a furred body. The Old Ones seem to have had a much more humanoid face, but *very* interestingly, the statue in Varlamore's museum depicts a Nagua *wearing* an Old One mask, which all of the Custodian Stalkers resemble to a varying degree. Likely, the actual Old One faces were more animalistic than we can see in their limited concept art.
Lastly, the Custodians are also the only things that drop the Alchemist's Signet. In addition to the Old Ones being masters of Herblore, the Alchemist's Society in Varlamore is said to have been founded on their left-behind expertise and teachings. The Signet would make no sense dropping from the Custodians - unless they have ancient ties to the Old Ones, and maybe carry some of their lost alchemical treasures.
So yeah, my theory is that the Custodian Stalkers are the intended vessels for the Old Ones' immortality gambit, left as soulless husks compulsively sacrificing others to the Strangler variant that birthed them. Whether they are just miming out the last acts of their creators, or just doing what any Strangler organism does, or even still trying (in vain) to fulfill the Old Ones' plan, I believe they are inextricably tied together.