Fields of Balance is a friendly island without any enemies in Portal Knights. It is inspired by Asian ecosystems, and has bamboo forests on it. It is a great place to build a base, but it's a bit more tricky to obtain this island compared to the other islands without any monsters.
It's only possible to unlock the Fiends of Balance during the spring event, but if you unlock it during this time, you will have permanent access to it. The Spring event runs from January 19th until February 23rd every year, so you have about a month each year to get it. It's actually possible to get it outside of this time frame if you change the date on your system's settings to be in this time, so that's possible if you miss out on the event itself.
Anyway, you get the Fields of Balance as a reward for completing the entire Spring event quest chain. This requires you do collect things, build things, and kill enemies of two different event-only islands. The quest chain begins with an event called The Emperor's Courier, so look for that on your map.
We won't go into detail about all the quests here, since we have already posted a full guide to the Portal Knights Spring event earlier. So just go to that guide if you get stuck on any part of the Spring event quest chain at any point.