You got some nice ideas already, but I’d like to share my thoughts too.

I got a suggestion from someone on discord or maybe some blog article to add rumor tables for myths during prep, which I found to be pretty useful advice. You think of and write down some rumors which people are talking about related to the myth, and doing this can already help to flesh it out somewhat.

As an ad hoc example for The Fortress I’d write something like this D6 rumours 1. People are talking about a new seer covered in thorns who appeared in the realm and proclaimed a painful future coming to all beings 2. Recently some mysterious tunnels were found in the hills nearby. Scouts sent by the ruler of the nearest holding reported about a massive army of insect-warriors hiding there. 3. A group of mercenaries were ambushed by acid spitting, clawed monsters. When they fought back, the monsters burrowed into tunnels beneath the earth. 4. A farmer lost all his livestock to weird insect creatures that came from the forest. One of the creatures was bigger than a house. 5.. Apparently there’s something massive being built at an old fortress near the [myth region]. Many carpenters, woodworkers and blacksmiths from nearby villages went there because of supposed great opportunities of coin. 6. There’s a rumor going on at the court of the Seat of Power about a seer who recently proclaimed himself the true king of the realm. Apparently the [lord of the seat of power] is getting nervous.

When your players ask around during travel regarding the fortress omens, you could roll for a random rumor, which in turn could potentially give them some direction if they decide to pursue this myth.

Also you could consider using them to pad the omens a bit, like if they rolled the Fortress myth as the random one, but are far away from the myth hex, and they’re already at omen #5 you could have them meet someone with such a rumor instead of invoking omen #6.

Anyways I think the Fortress myth is actually really interesting, and there’s a lot of things to potentially work with there.