A massive ball of dark, writhing Chaos energy, the Chaos Vortex appears and attacks April several times over the course of her journey. It is also responsible for attacking the Mother (possibly contributing to her death), damaging the Wood Spirit, and was part of the assault on Marcuria. The Vortex appears to grow in size each time it appears.
As April works out after her discussion with Burns Flipper, the Chaos Vortex is actually Gordon Halloway's Chaotic side after the Vanguard experimented on him. When the Vortex appears for the last time in the Guardian's Realm, April uses a Magic Binding Potion on the Talisman of the Balance, which draws the Vortex into the Talisman. Later, she uses the Talisman on Gordon himself, reuniting the cold Halloway with his chaotic side.