sword of equilibrium
 

It turned to black after I got attacked and won the fight. What gives? Is this a planned event?

The sword of equilibrium (+5att) is an interesting weapon. And an annoying one, especially early on. The way it works, is that it starts with 50 morale, loses morale at the end of combat if the enemy troops included humans or elves, and gains morale if the enemy had undead or demons. At 0 morale it transforms into the sword of darkness (+5 att, +10 if night, -20% defense to humans and elves), which continues with the same rules as the sword of equilibrium (so at 100 morale, it turns back into equilibrium). At 100 morale, the sword of equilibrium turns into the sword of light (+5 att, +10 during the day, -20% defense to demons and undead). It is really nice later on in the game, but if you did what i did in my last game (noticed it in greenwort castle, and went "WOOT +5 attack sword" and saved up to buy it, only to have it turn to darkness because practically all of the early battles contain humans), don't despair. For me, clearing the cemetery was enough to convert it all the way up to sword of light with max morale, which is a very nice weapon, and can take the loss of morale from fighting humans a long time before it goes back to darkness.

After which quest you can buy the Equilibrium Sword? I find it when I played with the mage, but with the warrior I can find it.

It's random where you find it (Where you can buy it). I found it in Haas labyrinth on my first game.

Thanks for the hint :)

I noticed that when the sword is not equiped during a fight, its morale is not affected by the battle. Therefore one as just to unequip it for not critical battle against units of the wrong side.