The Spiritual Soul was the essense of life, as well as the source of spirit of the Mind.
Once the Spiritual Soul blended with the Martial Soul and the Martial Soul evolved into the Fighting Soul, the Fighting Soul would become indestructible and the Spiritual Soul, everlasting.
He would continue to live on even if his physical body were to be destroyed.
The blending of the Spiritual Soul and Martial Soul certainly did not imply a complete fusion of the souls into one. Instead, it just meant that the two would cooperate and complement each other.
The Spiritual Soul of Su Mo had evolved into an extensive spirit of the Mind and had entered his Devouring Martial Soul.
According to the information given by Flaming Ghost Sage, the Ultimacy was the medium through which the Spiritual Soul could blend with the Martial Soul.