This card is a very interesting Thief Skill, in that it is an almost entirely defensive card, while most thief skills are about either gaining a skill or hamstringing an opponent.

In this one, after an attacker has declared the attacker, any user of thief skills and targets a champion in the opponent's pool. Usually the target is either (1) another truly deadly attacker, or more likely (2) a champion that never goes into battle because of their value in the pool (like Necba the Wrathmaker (14/DU-chase), Hettman Tsurin (257/4th), or others). This could often be a means to get an attacker to call off an attack and end a battle. Halting calls back the champion to the pool and fully ends the battle (no more rounds of battle are allowed).

As nice as it is, I would only play this card in a thief focused deck that was focused on speed. However, it might be useful to most decks in the setting you play in is very combat oriented, and your deck is a speed, defensive focused deck.

One needs to look at their home environment to decide if you need it. I usually thrive in combat, so I wouldn't play it, and most often wouldn't care if I lost the attack. The times it'll be the most effective is against a person who just goes for combat's where spoils seem guaranteed (like a really built up Tithian (301/3rd) or Living Wall (58/PO) attacking).