Sinister Wood, also known as Glow Wood, is a wood type that could be seen growing during the 2015, 2016, and the 2017 Halloween Events, along with Spook Wood.
It has the same exact growth patterns as Spook Wood other than the rarity of it; out of 100 Spook trees, only one of these might spawn. Notably, it is the second densest tree in the game, commonly requiring 250 chops with a Rukiryaxe to chop down a fully grown tree.
Sinister trees use the same growth inputs as Spook, though it is modified to possess neon bark and interior (without any color change) and rarer. Thus, it is merely a misconception that Sinister grows differently than Spook. The planks also entirely possess a neon texture which emits a strong orange glow.
Its trees can be found on any part of the game named Slate, Ground, or Road. Slate is any cliff in the game no matter the color. The ground is typically grass that trees grow on in their respective biomes, and Road is any road in the game, with the exception of Safari roads (as they are named "Read" due to a typographical error).
It can still be purchased from other players, and unfortunately, it has been circulating the market as a target for duplication, which made its rarity and value significantly lower over the past few years.