I'm not super familiar with WorldAnvil, but am always up to learn about new things.
In looking over the alliance information, most of the plot and dynamics seemed really straightforward until you got into the history and demographics sections. I would love to know more about the cultures of the two tribes, why they are compatible with eachother but not the Firgenholt, and relative population sizes. Are they matriarchal? patriarchal? democratic? What were the philosphical beliefs that they shared? How do the tribes play a role into the world as a whole? You mentioned the Wahstrehite King early on, but I can't really see where he fits into the narrative.
I do like the formal alliance tie and history that you are creating with the marriage, and with the exchanging of symbolic weaponry, especially since it is those symbols that will likely stay remembered in legend and history as the alliance survives. You might consider naming the weapons that are used (is it a hand-axe? a short sword? a javelin?)
I also came across a few typos if you're just looking for some cosmetic fixes.
reamained should be remained (structure)
weilding should be wielding (assets)
I was also a little confused about the difference between the Sceadu, the Mundbyrdians and the Sundorwikians (spelling? I saw it different ways). I tried to look into the individual tribes for info but didn't get much information about the Sceadu. What I saw about the Mundbyrdians did clarify a couple of my questions though, and was very well done!
I'd love for you to post an updated link in the future if you keep building onto the world so I can see how it progresses.