!ask can Baron be affected by Ahri champion unit and/or Ahri legend

Ahri Legend (Nine-Tailed Fox): Yes. When Baron attacks a battlefield you control, the Legend triggers and gives him −1 Might that turn. The ability doesn't "choose" Baron — it programmatically selects the attacking unit with no choice made — so his "can't be chosen by enemy spells and abilities" protection doesn't apply (Rules 355.10.d, 756–757). Same reason Vex, Apathetic can still stun him.

Ahri, Inquisitive (Champion): No. Her trigger says "give an enemy unit here −2 Might," which requires you to choose a target. Baron can't be chosen by enemy spells and abilities, so he's an illegal choice for that ability — she'd have to hit a different enemy unit at that location (Rule 355.10).