Nightmare Fuel is a refineditem used in the ways of dark and ancient magic. It requires a Prestihatitator to prototype and costs 4 Dark Petals to craft. It can also be obtained as a drop from various mobs and objects.
Nightmare Fuel is used to craft various Magic items. It is also used to fuel Night Lights and Maxwell'sCodex Umbra (to summon Shadow Servants). Chester can transform into Shadow Chester by placing Nightmare Fuel in each of his nine inventory slots on a Full Moon.
If a character's sanity decreases below 40% of their maximum (100 for Wickerbottom, 80 for most characters, 48 for Willow, 30 for Wes, and varies for WX-78 based on the upgrade level), all Rabbits change into Beardlings and can be killed to gather Nightmare Fuel (along with Monster Meat and Beard Hair). An easy way to do this is by trapping a large number of Rabbits and then lowering Sanity to below 40%, after which they can all be murdered all at once while in inventory. However, be careful, as this is liable to trigger the Krampus by advancing the player's naughtiness level beyond the threshold. Unlike Beardlings, Beardlords do not drop Nightmare Fuel.
Alternatively, Nightmare Fuel can be acquired by killing Crawling Horrors and Terrorbeaks. Crawling Horrors are easier to deal with than Terrorbeaks; the latter don't appear until Sanity is very, very low, so it's possible to kill Crawling Horrors one at a time.
A safer and simpler way to get Nightmare Fuel is to capture Bees with a Bug Net and then release them at an Evil Flower patch. This will eventually cause more Evil Flowers to spawn, and their petals can be crafted into Nightmare Fuel.
Perhaps the most dangerous yet lucrative way is to venture into a Ruins or Cave and find a location with Nightmare Lights and Nightmare Fissures. When the Nightmare Cycle ends, all of the Shadow Creatures that spawned from the Lights and Fissures will die and drop Nightmare Fuel.
All variants of the Reanimated Skeleton drop 2-4 Nightmare Fuel when killed. When the Ancient Fuelweaver is killed, all Woven Shadows will turn into Nightmare Fuel. It can also be used to refuel certain items. Unlike in the Don't Starve DLCs, Nightmare Fuel no longer gets wet, and Shadow Creatures no longer drop Nightmare Fuel at the end of the Nightmare Cycle.
The term "nightmare fuel" is a common phrase on the Internet used to describe works of fiction, or scenes therein, that are extremely likely to disturb and/or frighten the audience.
When left to unsettle on the ground, Nightmare Fuel appears as shifting, writhing goo.
Maxwell, upon examining the Ancient Guardian, says "My my, the fuel has changed you." This implies that prolonged exposure to Nightmare Fuel can alter creatures in some significant way. This is also proven by the ability to turn Chester into his shadow form via Nightmare Fuel.
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