Chiller is a focus mutation that's mutually exclusive with Carrie and Solar Sponge. Players with Chiller can freeze items in their inventory (or others') to preserve them until the ice melts. They can also freeze enemies in combat, which damages and debuffs them based on their cold soaks (yeti fur boosts this effect). Players with chiller are passively colder, but gain stress when hot (needs citation). Also grants +1 cool.
Somewhere in Fort Griam, northwest of Weezer Dam.
The freeze attack can become quite a powerful ability at higher levels of focus, especially given how rare cold soak is. Being passively colder makes it a bit easier to handle hot areas, but only if you opt out of the Yeti soak mutations. If you do get Yeti Skin/Fur, you negate the passive cold, if that's something that would become a problem. The antifreeze vomit for preserving inventory items is somewhat useful-- at least you'll never need to carry around a kidney cooler.
Being colder all the time might become a problem, especially since freezing to death outside of the Weezer area becomes is quite likely. Yeti Fur cancels out such a problem, but then you've got the issue of not being able to go into hot areas at all, which may be worse. Not being able to get Carrie or Solar Sponge can be a serious problem for some, and taking up a mutation slot makes this a poor choice for an early mutation. You'll also take stress when hot (though that might be an effect of Yeti soaks that someone got mixed up, needs citation).