Episode 22: Caleb's Challenge
This episode first aired on Toon Disney on July 11, 2005. It is the first episode where Aldarn was ever featured in a preview, and he dominated it. It looks to be a great Aldarn episode! Here's a transcript of the preview:

Various scenes of the characters play in the background; multiple shots of Aldarn with spacy hypnotized eyes, Phobos with rose petals, Princess Elyon, and Caleb looking sad.
Aldarn: The princess is back! We're signing a peace treaty today! Do you have a problem with me signing it?! (Aldarn points at Caleb, his eyes still spaced-out) Beat me in a leadership challenge!
Scenes of Will and the girls looking shocked, and Cornelia and Hay Lin looking upset
Taranee: Elyon clearly hypnotized him.
Scenes of Aldarn with glowing eyes, Caleb fighting killer vines
Caleb: You're my best friend!
Aldarn shouting with glowing eyes
Aldarn: This is for the good of all Meridian!
Vathek yelling with an enormous mouth
Vathek: Begin!
Aldarn attacks Caleb with a staff, gets pushed off edge of cliff, Caleb reaches out to him
Caleb: Aldarn, no!

Also, because I now have a camera phone and not enough things to do with it, here are some really low quality screencaps from the preview.