'It's safe to assume that everything they have is inside the ring, guess I don't need to waste time with them.'
Ainz was sitting not too far from them, as he watched everything in amusement, it was quite funny to him that he was so close to them, yet they couldn't see or feel his presence. He paid attention while listening to their conversation attentively.
The reason why he decided to use shadow shroud was simply because he wanted to make sure they were not hiding anything that needed to be uncovered.
Once he was sure there were no secrets to uncover, and the ring was the only thing they had on them, he realized the ring was the main reason why they could steal from their target and go unnoticed.
So he simply stood up, and began to stroll toward the swordsmen of the mist, he stretched his hand as his spear materialized in his hand, and Ainz simply slashed in front of him creating a blade of wind that severed the heads of two of the swordsmen.
