Deathcap is one of the few plants you can find while exploring Hades 2. These plants grow in specific regions and can be missed if you run through an area too quickly. What makes Deathcap harder to find is that it prefers certain weather conditions.
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While the Deathcap plant has a set area, it will only appear if you enter the region at a set time. For a lot of things in Hades 2, this is completely up to random luck, and you’ll find yourself trying to find it without much luck on your side. However, knowing what conditions to look out for will make it easier as you progress through your runs.
You can find the Deathcap mushroom by exploring Erebus in Hades 2. However, while you can find it in Erebus and you don't need Harvesting Tools to pick it up, a Deathcap will only appear when it's raining. If there's no rain in Erebus, the Deathcaps won't be there and you won't find them during your run.
Erebus has a chance to have dry weather, without any rainfall, or it can have a heavy downpour. You should know whether or not it is going to rain when you start your run in Hades 2. Since Erebus is the first area, you should know pretty quickly if there is a chance for a Deathcap to appear. If you are really looking for this resource, feel free to give up and start over until you get an Erebus scroll with rainfall.