Time passed quickly. By now, Rimuru possessed five Stones—Time, Space, Mind, Power, and Reality. Only the last one, the Soul Stone, remained out of reach. However, the conditions for obtaining the Soul Stone were rather unusual, and its ability—to create, alter, and manipulate souls—overlapped with Materialization of the Soul in certain respects.
Still, despite the overlap in some abilities, the Soul Stone could also make up for certain shortcomings of Rimuru's Materialization of the Soul.
For example, although the Third Magic was also a miracle concerning souls, its emphasis lay more on alteration and solidification. It could completely transform any soul, turning a human into a dragon or a dragon into a dog. That was indeed a form of manipulation, but it could not control souls on a massive scale or command them to fight for him.
He could certainly solidify souls and create Heroic Spirits, but they retained part of their consciousness, and doing so consumed a considerable amount of mana.
The Soul Stone, on the other hand, was far more domineering. It could even erase a soul's consciousness entirely, allowing the spiritual body to exist purely in soul form and create a ghost army for Rimuru!
In a sense, the Soul Stone and the Third Magic focused on different aspects. They could complement each other and allow Rimuru to completely grasp the laws of the soul system. That was quite good.
The only problem was that obtaining the Soul Stone required sacrificing someone important to you. Emmmm, after giving it some thought, Rimuru decided it would be better to just have Thanos deliver it to him.
Rimuru had also considered whether being powerful enough would allow him to ignore that requirement and forcibly take the Soul Stone. But the issue was that the being who imposed this requirement