Sunnycide is an egg-like Shroob monster that was created during the Shroob invasion of the Mushroom Kingdom in Mario & Luigi: Partners in Time, in case something went wrong with their plan of preventing the Yoshis from helping the Mushroom Kingdom. This strange monster was stationed inside Yoob's belly, where it served as the guardian of the Shroob-created eggs holding the captured Yoshis. Sunnycide would watch over these eggs until they presumably hatched and became more monstrous Yoobs to conquer Yoshi's Island. Sunnycide also guarded the exit to Yoob's stomach, preventing anyone from leaving the intestinal factory. At last, Sunnycide was destroyed by the four Mario Brothers of past and present (Mario, Luigi, Baby Mario, Baby Luigi). Sunnycide acts as the last enemy fought on Yoshi's Island, and the game's second Boss.
Sunnycide resembles a large pink egg with green spots (similar to Yoob's egg). The shell is cracked through the middle horizontally, creating a mouth and separating Sunnycide's body into two halves. Two eye-stalks sprout from the top of the egg, while two tentacle-like arms and two round feet emerge from the bottom half. Inside the mouth is Sunnycide's weak spot, a large bump partially covered with a bandage.
After the Mario brothers helped a band of Yoshis through the factory in Yoob's stomach, the heroes eventually enter the lair of Sunnycide. The monstrous egg creature immediately attacks the Mario Bros, beginning the inevitable battle, though Luigi keeps trying to run away but is repeatedly blocked. During the confrontation, Sunnycide's primary attack is picking up the Yoob eggs containing Yoshis resting in his lair and flinging them at a Mario brother, using a targeting reticule similar to Yoshi's when he throws a Yoshi egg. The Mario brothers can Jump over the eggs to dodge the attack. When the monster runs out of eggs, Sunnycide will slam the ground and five more eggs will fall from the ceiling. Sunnycide will also attempt to inhale a Mario Bro. and eat him. As the Bro. moved closer, the corresponding Baby Bro. can use his Hammer to strike Sunnycide's weak point inside its mouth, thus countering the attack. If the brother is eaten, Sunnycide will recover some HP and spit out the Bro. that was eaten, injuring him in the process.
Since Sunnycide is a hard egg shell, most attacks are useless against the egg monster as they only do very little damage, although it suffers from a weakness against fire. To defeat Sunnycide, the Mario Bros. must find a way to attack the monster's weak spot inside its mouth. To do this, Mario and Luigi break open the Shroob eggs used by Sunnycide as weapons and free the Yoshis trapped inside. After a Yoshi is freed, it runs up to a ledge overlooking the battlefield and attempts to aid the Mario brothers by pushing a large Chomp Rock on top of Sunnycide. Unfortunately, one Yoshi is not strong enough to push the Chomp Rock off the ledge. Five Yoshis are needed to push the Chomp Rock onto Sunnycide. Incidentally, every set of five Yoshis has one green Yoshi, one black Yoshi, one blue Yoshi, one light blue Yoshi, and one pink Yoshi.
After the five Yoshis push the Chomp Rock off the ledge, the huge stone knocks Sunnycide on its back, exposing its weak point (the Yoshis would then run off to safety instead of staying and fighting). However, Sunnycide is not finished attacking the brothers yet. While on its back, Sunnycide can perform two moves. First, Sunnycide can pull a chain, creating a hole in the floor. To dodge this attack, Mario or Luigi has to jump right before the hole appears. The hole will quickly disappear, allowing the Mario brother to fall on solid ground. If the Bro. fell through the hole, he would again inexplicably fall from the ceiling and hit the floor, getting damaged in the process. Additionally, Sunnycide can pull a rope that turns out the lights in Yoob's stomach. This technique will also summon a Boo to attack either Mario or Luigi. Although dim, the Boo can still be easily seen, allowing the Baby Bro. to block the ghost's attack with a Hammer strike. The side of the chain or rope corresponds to which Bro will be attacked. You can find the Bro he is going to attack by looking at which arm he used to pull it. His right arm goes to Mario and his left goes to Luigi.
Eventually, Sunnycide will manage to get back up and continue throwing eggs at the Mario brothers, and so you must free more Yoshis to knock Sunnycide back down. After repeated attacks, Sunnycide will brilliantly explode and any remaining eggs break open, releasing the Yoshis trapped inside. With the egg guardian destroyed, the freed Yoshis, the four Mario brothers, the captured Toadbert, and the previously imprisoned Baby Bowser escaped through Yoob's intestines back to the safety of Yoshi's Island.
Also, in Mario and Luigi Bowser's Inside Story, Sunnycide appears behind a Dark Fawful Guy which is found in Peach's Castle.
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