The Summerween Trickster is a monster that appears in the Gravity Falls episode "Summerween".
In the town of Gravity Falls, Oregon, the holiday of Halloween is celebrated twice a year, once at its normal time of October 31st and also in June during the American summertime. At this time the holiday is called Summerween and entails all of the usual traditions of Halloween such as scaring one's neighbours, putting up spooky decorations and the most beloved practice of trick-or-treat. A local legend of Gravity Falls tells of a mysterious being known as the Summerween Trickster who seeks out children lacking in Summerween spirit and devours them.
As Dipper and Mabel Pines discover on the night of Summerween, the Summerween Trickster is quite real as he knocks on the front door of the Mystery Shack. Thinking that this stranger is too old to be trick-or-treating, Dipper turns him away, only to incur his wrath as the Trickster demonstrates his power by eating a child whole in front of the Pines twins. Having been insulted, the Trickster threatens to kill the children unless they can bring him 500 pieces of candy before the last Jack-O-Melon is snuffed out.
Dipper and Mabel, along with their friends Soos, Candee and Grenda, go out trick-or-treating and manage to fill up a wheelbarrow full of candy. However, Dipper accidentally sends the wheelbarrow rolling down a hill and into a polluted pond, ruining all of the candy. Time runs out and the Trickster appears to receive his treat, but with the candy lost the Trickster decides to take the children's lives. Soos and the kids flee in Soos' pickup truck, but the Trickster transforms into a huge amorphous creature with many spidery limbs and gives chase, cornering the group inside a convenience store.
Inside the store, the Trickster grabs Soos and swallows him whole. The children try to fight back with various toy weapons and with each strike a tiny fragment of the Trickster's body breaks away. The kids discover that the flakes are actually made of stale taffy, which is when the Trickster removes his mask and reveals his true nature: he is made out of cheap, low-quality candy that nobody likes. As the Trickster moves in to devour the children, Soos suddenly erupts from the creature's belly having eaten his way out. As Soos eats away at the Trickster's candy body, the Trickster weeps tears of candy corn, happy that someone actually thinks he tastes good. The Trickster then dies as Soos continues eating, and the child that the Trickster had eaten earlier emerges alive and well (apart from being traumatized).
The Summerween Trickster appears humanoid at first, wearing a patchwork coat, a pointed hat and a smiley-face mask. These garments and his seemingly spindly frame give him the appearance of a scarecrow, but when the last Jack-o-Melon candle is extinguished, the Trickster transforms into a huge swollen form with numerous limbs, making him look like a giant spider.
The Trickster's body is actually made up of old discarded candy, mostly low-quality confectionery that children generally dislike: black liquorice, saltwater taffy, stale salty chocolate, candy corn and cheap waxy jelly beans.
The Summerween Trickster demonstrates that he is capable of altering his size and shape, though this likely depends on how much candy makes up his body.
The Trickster is made of candy so he is entirely edible (or at least as edible as liquorice and candy corn can be).