From the Super Mario Wiki
X-Yux is an upgrade of the original Yux found in Paper Mario: The Thousand-Year Door. According to Goombella, its name is pronounced "Cross-Yux" instead of "Ex-Yux", although she herself opens her tattle with "That's an X-Yux", suggesting that she herself used the incorrect pronunciation.
A solitary X-Yux is used as the guard of Grodus' room in the X-Naut Fortress. Also, if Mario answers three questions wrong to the Thwomp in the fortress, he will send out two X-Yuxes to attack Mario. In battle, a X-Yux spawns two Mini-X-Yux each round, up to a maximum of four per X-Yux. As always, it's invincible while Mini-X-Yux are surrounding it. When an X-Yux is surrounded by four Mini-X-Yux, it shoots five beams at Mario and his partner.
The X-Yuxes, as with the rest of the Yux family, are notable in that their beam attacks pierce all Defend Plus and Damage Dodge badges that Mario and his partner have equipped. This means that their attacks can only be defended against with the use of Guards and Superguards. And the immobilization can only be blocked with Superguards.
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