Divinity.

The Tree Father said to them, "It is done. The next step is to upgrade the divine spark into a world forge." 

The others looked on with calmness, but they were all excited and happy. They have taken another step towards perfection. It is something to be happy about. 

Legion-unity said with anticipation, "It is time for you to join unity." 

This is why he has been looking forward to the Tree Father becoming a divine spark. He wants to gain something powerful to improve himself and Legion.

The Tree Father will be the next one to join Unity after Aeternus. Aeternus granted them immunity to Chaos energy, so all of them won't be hurt by Chaos energy anymore.

They can finally allow Chaos energy to pass through the mind network. They don't do so to keep Legion-8 and Helios safe. Now the tree father can also join unity since he has gained immunity and perfect control of his godfire.