The target opponent chooses whether to have Pia’s Revolution deal 3 damage to them 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.

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.

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.

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

There are many important moments in the story, but the most crucial—called “story spotlights”—are shown on cards. These cards have the Planeswalker symbol in their text box; this symbol has no effect on gameplay. You can read more about these events in the official Magic fiction at http://www.mtgstory.com.