OFB-11!

I rewatched all the saves to understand what happened. Dave played better. In the first week, he sacrificed building Pyres for tempo, upgraded Storms early, and started clearing the treasure map on day 116. Even with that, on day 122, I wasn't too far behind, but day by day, his skill made a difference. Two things saved me. 1) RNG didn't place a control artifact on the map, so when Dessa came to kill me, he couldn't reach my nearest town for just one movement point on two turns. This gave me time to retreat and look for something else. 2) In his last turn (135), the opponent clearly made a mistake. His secondary hero, Monere, moved to the front line, captured my nearest town, but didn't kill three of my heroes (with maximum speed) who were building a chain around that town. In my next turn, I used this chain to take back two towns, kill the army on the secondary hero, and kill the main hero, Dessa.
I was definitely trying hard, and I had trained mirror matches a lot beforehand, but, to be honest, it was a lucky win.
P.S. Ah, and I messed up in the final battle. As soon as I felt like victory was real, I made a stupid, noob-like play.

Thanks for the game and for respecting my time.