Commander Liara Portyr
Legendary Creature — Human Soldier
Whenever you attack, spells you cast from exile this turn cost 
less to cast, where X is the number of players being attacked. Exile the top X cards of your library. Until end of turn, you may cast spells from among those exiled cards.
"It's a ghoul-eat-you world out there. Follow my orders if you want to survive."
Illustrated by
Commander Legends: Battle for Baldur's Gate#270UncommonEnglishLegality
Rulings
If you attack a planeswalker but don't also attack that planeswalker's controller, that player won't count toward the value of X.
Commander Liara Portyr doesn't have to be among the attacking creatures.
Commander Liara Portyr's triggered ability reduces the cost of all spells you cast from exile, even if you are casting them with a different permission. For example, if you are casting an exiled permanent spell with an Adventure in the same turn that this triggered ability resolved, it would cost less to cast.