Also known as Batsu tamago (Punishment Egg).
X-Eggs are known to say muri which means impossible or useless.
Should the child with the Heart's Egg become burdened by worries and doubts, the Heart's Egg will slowly become tainted with a black color. If the child gives up his or her dreams instead of pursuing them, it will eventually become an evil and mischievous X-Egg and attack everyone in sight when it hatches into an X-Character.
Some X-Eggs however are quite friendly such as the ones that stayed at Rikka's apartment building. They cared about Rikka and explained "how they had a lot of fun with her" after they were purified by Pure Feeling and Amulet Diamond.
X-Eggs can also build up a lot of stress and eventually self destructs due to its extreme negative emotions. Not only that, but when enough X-eggs are around in an area, they can perform formations that can be quite challenging to win against them, or they can combine powers to create a "bigger X-Egg" and hatch to become a "Mega X-Character" which can be EXTREMELY dangerous.
Only Amu Hinamori, Utau Hoshina, and Rikka Hiiragi have been able to purify the X-Eggs, along with an attackYamato Maihime and Clown Drop can use together called 'Queen's Waltz'. Platinum Royale and Amulet Heart can also use 'Platinum Heart' to do the same.
X eggs are pitch black with a big white X on it. An X character has dark purple skin, has a big red X on it, and has white eyes and a white mouth.