A bit confused about some cards. Dragon Thief and The Shadow of the East. Both say choose and discard attachment, does this have to be one you control, one your opponent controls or can it be any attachment in play ? Should it not say opponents if it affects opponents ? Just a bit confused about the wording I guess.

Toqtamish said:

It could be one that you control or your opponents control. If it could only be an attachment you control it would say so. Same goes for if it could only discard an opponent's.

What I'm thinking you may be getting confused by is costs. To pay the costs of triggered effects you must use cards/gold/influence you control. If Dragon Thief instead said, "Response: After Dragon Thief comes into play, discard an attachment to discard an attachment your opponent controls." you would have to first discard an attachment you control (the cost) to be able to discard an opponent's attachment (the effect). You can generally tell if an effect has a cost if it's written in the "Do X to do Y" format. If you look at Dragon Thief and The Shadow of the East you don't see "Do X to do do Y." Therefore, there is cost to trigger the effects of Dragon Thief and Shadows of the East. All you have to do to be able to trigger the effects is play the cards they are on legally.