@Wataterp
From what I've seen, I think "archmages" are people with high mana or mana control, basically just better tuned to use mana, and they are just archmages from birth. During Ernon's flashbacks, we saw him with the violet eyes as a child, which is why he was picked up and practically enslaved in the first place. Maybe Tassel was an archmage from the start and no one noticed? This is all off the top of my head, so I don't really know if it's accurate, but it still might be true??
Edit:
I recently re-read this manga and I noticed that there was a few panels where Tassel had violet eyes. It was only about 2-3 of them, and it was before we were told what violet eyes signified. Adding onto this, when Ernon yells that Tassel is his army's own mage, he also said something along the lines of, "You have to keep some secrets during war!" (off the top of my head). Another thing is, people seem to reference archmages and mages as different things. I think this is enough evidence to suggest that "archmages" are naturally born with higher than average magic ability and have violet eyes to signify that.
Edit again pog:
@satomobileot5
I the first edit answers your question, because of the foreshadowing in the earlier chapters to Tassel himself (who is probably the real one) having violet eyes probably means he was just keeping a secret... But shapeshifting would be cool too, sounds like a smart tactic to throw off the opposing side.