While exploring the ruined world of Thymesia, you'll encounter many dangers, such as corrupted enemies and even an eerie yellow fog. This fog will slowly poison you over time and can kill you very quickly if you haven't put many levels into your vitality. This guide explains how to clear the yellow fog in Thymesia, so you can open up the map a little more.
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To clear the yellow fog from an area, you need to destroy the large pulsing yellow pustule that should be somewhere close by. The pustule emits the fog, so by destroying it, you'll remove the fog from that area. To do this, you can swipe at it with your melee attacks or use ranged feather attacks to cause slower damage from a distance. However, you'll likely need to walk through the fog to get to this pustule, meaning you'll become poisoned quite fast and could die. It's usually better to get to a ladder or move through an area of yellow fog quickly instead of trying to take out the yellow pustule.
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Early on in the game, there's little you can do to increase your defense against the slow poison that the yellow fog causes. In most areas, there will be a quick exit that you can reach somewhere in the fog, usually a ladder. It's better to run to these and progress than it is to remove the yellow fog. In some cases, the pustule producing yellow fog will be hidden on another level above or below you, so you may need to get through the fog and climb a ladder before removing it to get into the next area. If you can't see a route forward through the fog, then you'll need to find the nearby ladder or path out of the area until you stumble upon the yellow pustule.