From the Super Mario Wiki
A X-Yux (pronounced "Cross-Yux" instead of "Ex-Yux") is an upgrade of the original Yux found in Paper Mario: The Thousand-Year Door. It is the one guard of Grodus' that he trusts to protect his personal room in the fortress. Also, if Mario answers three questions wrong to the Thwomp in the fortress, he will send out two X-Yuxes to attack Mario. It 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.
Although X-Yux is to be read "Cross-Yux", Goombella uses the word "an" before saying the creature's name, indicating that she is pronouncing it as "Ex-Yux".
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