Entirely Normal Armchair

During your turn, if this card is in your hand, you may hide it on the battlefield.
{0}: Return this artifact to its owner's hand. Only your opponents may activate this ability and only if they see this artifact.
{2}, Sacrifice this artifact: Destroy target attacking creature.

Unsanctioned

During your turn, if Entirely Normal Armchair is in your hand, you may hide it on the battlefield.
{0}: Return Entirely Normal Armchair to its owner's hand. Only any opponent may activate this ability and only if they see Entirely Normal Armchair.
{2}, Sacrifice Entirely Normal Armchair: Destroy target attacking creature.

Unstable

During your turn, if Entirely Normal Armchair is in your hand, you may hide it on the battlefield.
{0}: Return Entirely Normal Armchair to its owner's hand. Only any opponent may activate this ability and only if he or she sees Entirely Normal Armchair.
{2}, Sacrifice Entirely Normal Armchair: Destroy target attacking creature.

Entirely Normal Armchair can be hidden in plain sight (i.e. just sitting there).

You must legally get Entirely Normal Armchair into your hand before you can attempt to sneak it onto the battlefield. For example, you can't sneak it in from your sideboard or collection during a game.

Your opponent may activate Entirely Normal Armchair's first activated ability even if they can't literally see it, provided they can identify where it is. That is, you can't just put your hand over it and claim that can't see. Well, you can claim that, and it might be true, but it won't do what you want it to.