How to Unlock the Trial of Chaos

The Trial of Chaos can be found in the Chimeral Wetlands, which is accessible in Act 3. You won't be able to Path of exile currency take the Trial, however, unless you have the Chimeral Imbued Ultimatum. This is the Chaos equivalent to Balbala's Coin and is dropped by the boss Xyclucian, the Chimera.

You can find Xyclucian in the Chimeral Wetlands as well. To find him, continue exploring the area until you start hearing an ominous roaring sound. That's when you know you're close to Xyclucian.

How to Beat Xyclucian, the Chimera
Xycluxian, the Chimera, is a large, dragon-like creature that can command all elements. And, unfortunately, it is not afraid to use them. You will face every elemental challenge possible throughout this fight, such as ice, fire, lightning, and even poison. Gearing up with some strong elemental resistances is a must before taking down Xyclucian.

Thankfully, the boss is a little more on the slow side, especially when it lands and stays stationary for a few seconds. It's not long, but it is long enough to land a couple of POE currency buy hits before the beast rises again.

The biggest tip you can use to your advantage in the Xyclucian fight is destroying the rock pillars. There are a few of these rock pillars situated around the arena. When Xyclucian perches on one to unleash whatever elemental attack it has up its sleeve, start focusing your attacks on the pillar instead of the beast.

These pillars can be broken, sending Xyclucian into a rocky hole and trapping it there for a decent amount of time. Take that opportunity to get in as many hits as possible without worrying about retaliation.