I'm sure archival gave a better answer but this is how I look at it The undying mind was the axis mind of the sol divisive, and its goal was to protect/reclaim the black garden as well as lead the sol divisive subtype Now that it and the raid bosses are dead it's possible that the sol divisive don't have any proper form of leadership and are now operating on their last given orders/prime directives

<The Undying Mind was made as a defender of the Black Garden. Its purpose is to usher it back into the abyss, as the Garden anchors itself to an area when it is not awake, as it anchored itself to Mars when the Heart of the Black Garden died. The Undying Mind is capable of death, but reawakens when the Garden does. This happened when the Lunar Pyramid reached into the Garden and brought it back to life, removing it from Mars and renewing the Sol Divisive Vex's faith. The Undying Mind used this to open a Vex gate on the Moon and send the Divisive through in order to secure the Pyramid for themselves. When the plan was foiled, the Darkness watched and adapted accordingly to our strength, as we were unaware that it was another test, like the Garden of Salvation raid and the Heart of the Black Garden before it.>