If a nontoken artifact is put into your graveyard at the same time as Pia's Revolution, Pia's Revolution's ability triggers.

It doesn't matter who controlled the artifact when it was put into your graveyard. Pia's Revolution's ability will trigger if you own that artifact.

The target opponent chooses whether to have Pia's Revolution deal 3 damage to him or her as its ability resolves. You won't return the card to your hand if that player chooses to be dealt damage or if the card leaves the graveyard before the ability resolves.

If there are no legal targets for Pia's Revolution's ability (perhaps because each of your opponents has hexproof), it will be removed from the stack with no effect. No one may choose to be dealt damage and you won't return the artifact card to your hand.

There are many important moments in the story, but the most crucial—called "story spotlights"—are shown on cards. You can read more about these events in the official Magic fiction at http://www.mtgstory.com.