Atlas cast a shroud on himself to make him invisible before attacking.
It was one of the spells that Lilith told him he must learn. It didn't matter if he learned anything else or not because she knew that this spell could save his life when he needed.
This spell was restricting for Lilith since she couldn't use the shroud while using another spell. While she was under the effect of shroud, she was no less than an ordinary human.
Unlike her, Atlas didn't depend on spells as much as he did on his strength. Even if he used Shroud, he could still fight at his full capacity. He didn't have to worry about canceling his Shroud to fight back like Lilith.
For him, the shroud was a versatile skill that gave him an advantage that he wouldn't have had otherwise. The only flaw was that he couldn't use a shroud for longer durations and that it placed a burden on his soul.