(This is for the fanfiction "Crossover Together". Info may change from canon information)
Shukaku is one of the nine-tailed beasts that had taken residence within Gaara's body.
Shukaku is a sandy-brown coloured tanuki, with black (dark blue in the anime) cursed seal markings all over its face, body, and tail. It has a jagged, concave mouth with no tongue, and the sclerae of its eyes are black, with yellow irides and pupils that each takes the shape of a black four-pointed star with four black dots around it. During the last days of the Sage of Six Paths, Shukaku in its youth looked the basically same as it does presently, only much smaller with shorter ears and more spaced out markings.
During Gaara's youth long ago, Shukaku remained as one of the last Nine-Tailed beasts, and he managed to escape from death by possessing his body, making Gaara a "cursed" individual of the Fanalis.
In an attempt to remove Shukaku, Sombra went into Gaara's subconcious and confronted Shukaku directly, the Tanuki knowing Sombra by name. when asked about it, he claimed that he can hear and see everything Gaara sees, and all information he had gathered, Shukaku gathered too. When offered to leave, he joked on how he could get out anytime he wanted to. He then asked the state of Tirek, who he considered as a pathetic little troll. He then joked about Gaara's one-track mind, a trait he had since birth, and that he had forced Gaara to stay awake so he wouldn't be completely possessed by him. He revealed to Sombra that all of Gaara's sand manipulation was actually him, and he decided to strike a deal with Sombra. What the deal was is unclear.
After Heine Wittgenstein spoke with Gaara, Shukaku finally took full control over Gaara and released himself onto the Surface World. He openly mocked the team, and thanked them for making his escape easier. He proceeded to assault them all, and tried to kill them on sight as his first act on the Surface World. Grell Sutcliff and Aladdin began to lure Shukaku through the forest, over towards the shores of the nearby sea. At this point, Shukaku swallowed Grell and Aladdin whole, but soon after Gaara actually took control over Shukaku, and force him into the water. Shukaku regained control too late, and ended up landing into the water, dissolving and drowning in the process.