So I played a combo last night, using a card to give one of my opponents characters the Night's Watch trait and then playing Yoren's Task to discard him to search my deck for a nights watch character. Am I aloud to do this or can I only discard my own cards?

Wheelz said:

You may only pay costs using cards that you control, and the discard on Yoren's Task is phrased as a cost. So no, sorry, you cannot do it that way.

Wheelz said:

Discarding a Night's Watch character is a cost to trigger the effect of this event. You may only pay costs using cards you control. Therefore, you cannot discard an opponent's Night's Watch character to pay for this event.

"Do X to do Y" is the common template to use when it comes to paying costs for effects. X is the cost, and it must be paid by you using cards you control. Keep in mind that not all effects have a cost.

I hope this helps.

Thanks for that, I thought I was pushing it a bit. But if I took control of that card through an effect or response first then I could discard it?

Wheelz said:

Sure. You can do as you please with cards you control, even if your opponent owns(came from his deck) it. You can choose it for claim, to pay for effects, etc.