Basilisx is a character in Super Mario Bros. Z made by Alvin-Earthworm.
Basilisx is, as Bowser calls him, his greatest soldier. He has an uncanny ability to turn people into stone by staring at them. He has the ability to retract and extend three claws for each arm, similar to Wolverine from X-Men. He assisted in the assault against the Sky Pop. Then he attacked Mario to avenge a friend and has him pinned down on the ground, ready to kill Mario, when the episode ends.
Not much is known about Basilisx, other than he may be the result of a super soldier project conducted by Dr. Eggman.
He is much faster, stronger, and an overall better fighter than any of Bowser's soldiers, even able to defeat and poise to kill Hammer Mario (though it should be noted that to do this, he used the stone encased Luigi as a way to halt Mario's attack, and use two poison mushrooms to remove his hammer form and severely weaken him).
Basilisx's raspy voicing and speech, the play on words "this demonic supersoldier" on Episode 8, his shadowlike appearance, and his vicious fighting suggests a demonic personality. However, he claims to have deep sadness regarding the fatal lava-scalding of his "besssst friend", Koopa #3 of Room 2 in Bowser's Castle. However, this may have been an untrue excuse to kill Mario because there is no lava in Room 2. (Technically, there are no koopas in Room 2, and there isn't a Bowser's Castle.)
According to Alvin-Earthworm, Basilisx will be omitted from the remake. It was stated that while Basilisx was well-received by the fans, the creator had felt Basilisx served no purpose from the lack of background storyline and history in the franchise.