I saw that it said
However, Shadow Force provides a good deal more power, so much so that Lugia and Giratina risk being OHKOed by Shadow Force after taking Stealth Rock damage provided Arceus has used Swords Dance once, is Adamant, and is holding Life Orb.
Shadow Force also alleviates any fears about Wobbuffet. Unfortunately, Shadow Force's charge turn makes it easy for the opponent to play around.
But I didnt see that arceus could learn Shadow Force. what's going on?
Arceus can learn shadow force. But it is through an event back in Diamond and pearl, not by leveling up. So it does not appear on the Learn list. (on the DB anyway). But it is usable on Pokemon Showdown! which most people use, so they decided to include it.
