And a year later I read the thread...

I think you are right about "Tellert" and "Tefferti" being way too close to be coincidence. Likely Ms. Coakely misremembered the name or misread it at a quick glance. I would consider it to be two versions of the same character with "Tellert" being the shortened or common form.

That said I think "arcane warriors" is the big clue. Per TWC:9, arcane warriors were established by Skelfer around 885/6, and rooting out evil undead sorcerers is just the sort of thing they were made for. So I think we are looking at an event that took place early in the 4 year war Skelfer waged against the Mage Kings between 886-900 per TWC:11. Could be a couple years after that, but not much I don't think.

Tellert could have arisen any time before that since he seems to have kept to himself in some remote part of the Redwood nearish to Jackport. Usually "ancient" to me implies something more than a few centuries, so I personally would put his origin back to at least the reign of the Bloody Duke and the Temple of ID period, but sometime after the Mad Wizard.

Very interesting guesses, in my opinion. This is a theme and a topic that I like.