Dynasty weapons.

By the way, it's easy to upgrade dynasty weapons. Choose a completed campaign ("I chose map 4 for the demons"), and start the playthrough again. When you've gained strength, look for areas with more neutral creatures, but don't attack them; just skip turns for about 25 months (press the E key) – it doesn't take long. Check the neutral creatures' legions, buy your own troops, and choose the weapon you need to upgrade. Defeat the neutral creatures in auto-battle. Done, the weapon is at its maximum level. Choose another weapon and move on to the next neutral creature. It takes 30 minutes to upgrade 22 dynasty weapons.

P.S. Also, please correct the name of my favorite weapon to "Sword of Wiselbon," not "Sword of Vibslon."