The first technique was the most vital technique for King Envy's rise, the soul absorption technique. The technique had already been explained in the introduction, but on a further note, the technique was only able to be used on those weaker than you in soul strength. You could absorb the soul of those stronger than you but if you did it would potentially overwhelm your soul and result in permanent damage! Lucius, however, knew that he could probably use it to absorb the souls of those stronger than him as he had created a far stronger mind palace than others and because of the potential empowerment from the two infinity constructs.