Lucius was currently using a type of soul magic that was similar to the one he had used before, the resonance spell. This was called as the Soul sense spell and could be said to be a lower version of the same category of the spell.
While the other one could attract the wandering souls, this one was used to find soul remnants and other such constructs. This consumed a very small amount of soul energy and thus could be used without him taxing his soul too much.
The last time Lucius used his soul energy was when he gathered the wandering souls of the fake bandits to refine soul essence. After that, it had taken him over a month to recover it all.
White the soul remnant locked on, Lucius started to pull it towards him using the soul sense. Since it was just a remnant of a soul, it had no will and thus could be easily controlled.