Kingswood Trail says " Response: After you declare attackers during a POW challenge, put Kingswood Trail into Shadows to choose and kneel a character with a POW icon controlled by the defending player. "

I was wondering can I use this ability (to put it into shadows for further use) if there are no characters I could kneel? They are eighter knelt already or they are unable to kneel because I used a Black Cells on them ( Response: After a card comes out of Shadows, kneel The Black Cells to choose an opponent's character. Until the end of the phase, that character cannot kneel or stand .)

There are two different answers depending on the situation.

If your opponent controls no characters with the POW icon, you cannot trigger Black Cells and put it into Shadows because there are no legal targets. If you cannot choose a target that the effect tells you to "choose," you cannot trigger the effect.

If all the characters with the POW icon that your opponent controls are already kneeling, you can trigger the Black Cells and put it into Shadows if you like. The effect does not say to choose a standing character with the POW icon, so you can choose a kneeling character with the POW icon and let the effect fail.

Thanks, thats exactly why I got confused again, so if I can choose the target (a char with a POW icon) it is ok. The fact he just can kneel or is already kneeling does not matter.