Chapter 54 Dark Star.

When Xaro attacked Lucifer some time back, Lucifer used quick thinking to create clones. The clones created by Lucifer were unique clones with his will inside them, so when they were destroyed by Xaro's Aura, Lucifer's will got mixed with his aura. As Xaro was low on the essence, his defense of consciousness was too weak, so the will easily entered his soul. However, as Xaro's soul was in round 2, it took some time for the will to act up.