Will we ever get the full history of Stagsi?
You’ll probably never get the full story on Stagzi because of the way roleplaying games tend to work.
Clearly Trelle and Stagzi have a previous relationship, but the two of them already know what that relationship is, so they don’t spend a lot of time explaining it to each other.
So it would take an outside person to ask for story or clarification. But as long as the other characters never say, “Hey, wait a minute, what do you mean ghost army?” we’ll never know any further details.
Think about it from a different standpoint: When Trelle asked how the party knew Prince Elk, she didn’t get a full explanation, she got just enough to keep the scene moving. But as an audience it doesn’t matter because we saw how Elk and Ket met.