Set

Set is one of the slow game actions you can perform on your Main Phases (alongside Summon).

Set means playing a card from your hand face-down onto an empty active zone you control in the Battlefield. Your opponent doesn't know what it is until it is revealed through Flipping (or some other means).

You can set both Creature and Artifact cards, but not Monuments.

A set card must be flipped when it is targeted. In addition to that, Artifact cards can be proactively flipped at instant speed, even as part of activating its effects. A Creature card can only be flipped when targeted (either for an effect or attack; either by you or your opponent).