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 full damage will be taken if no guard is performed, and only 1 damage will be blocked with a regular Guard, no matter what. However, the Superguard will, as always, prevent all damage. The Immobilization side effect of the beams can even pierce through a Guard, so one should attempt to Superguard against this foe whenever possible.
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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