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. Sunnycide's name is a pun on the term 'sunny-side up/down', ways to fry eggs, and the suffix -cide, meaning "to kill." It 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 it. Sunnycide acts as the last enemy fought on Yoshi's Island, and the game's sixth 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 resembling an egg yolk 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. It immediately attacks the Mario Bros., though Luigi keeps trying to run away but is repeatedly blocked.
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 it 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. The corresponding Baby Bro. can counter the attack by using his Hammer to strike Sunnycide's weak point inside its mouth. If the brother is eaten, Sunnycide will recover some HP and spit out the Bro. that was eaten, damaging him in the process.
Since Sunnycide is encased in a hard, outer shell, most attacks do very small damage to it, although it is weak against fire. To defeat Sunnycide, the Mario Bros. must 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. Five Yoshis are needed to push the Chomp Rock onto Sunnycide. Incidentally, every set of five Yoshis has one green Yoshi, one black Yoshi in the American English version or one yellow Yoshi in the Japanese, British, French, German, Italian and Spanish versions, one blue Yoshi, one light blue Yoshi, and one pink Yoshi.
After the five Yoshis push the Chomp Rock off the ledge, the Chomp Rock knocks Sunnycide on its back, exposing its weak point, as the Yoshis run off to safety. 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 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 Boo's attack with a Hammer strike. The side of the chain or rope corresponds to which Bro will be attacked. The player can tell which Bro he is going to attack by looking at which arm he used to pull it. His right arm means Mario will be attacked and his left means Luigi will be attacked.
Eventually, Sunnycide will manage to get back up and continue throwing eggs at the Mario brothers, so they must free more Yoshis to knock Sunnycide back down. Once Sunnycide is defeated, any eggs remaining in the battle break open, releasing the Yoshis trapped inside. With Sunnycide defeated, the freed Yoshis, the four Mario brothers, the captured Toadbert, and the previously imprisoned Baby Bowser escape through Yoob's belly back to Yoshi's Island.
Names in other languages