After the black liquid fused with the Dragon Pulse, it used the life energy that connects everybody to spread to all human beings; not to anyone or anything that has a consciousness.
It acted like a virus in the way it spread. However, it did not have any effect on people after entering their bodies. Truth be told, it was very hard to be discovered as it blended with people's life force like it was a part of them.
While this virus was quickly spreading from the Central Area of Amestris, Edward had a map in front of him to monitor the spread; he could control the speed of the virus spreading.
However, he did not rush. At this stage, everything was a test; he wanted to see whether Truth will discover him and intervene. Anyway, he was nothing but a clone at present, so he did not care if he was killed.
Worst case scenario, he was discovered and he was forced to flee.
While Edward was slowly testing the water and waiting for his opponent to respond, Truth was quite busy.
Although it was surprised at Edward's use of void energy, it thought that the wizard was being paranoid and wanted some privacy. So, after leaving a clone to keep watch on the house, Truth started to deal with his own plans.
It entered the Knowledge Dimension where the Eye of God resided. The eye is a form of artificial intelligence created by Truth to oversee the dimension; it is similar's to Edward's Morgana.
The only difference is that the "eye" does not have any emotions or thoughts; it only followed Truth's orders. The reason that both Father and Edward were able to learn the "Knowledge of God" was because Truth allowed it.
After entering the dimension, Truth soon came in front of a Gate. Despite its non-existent feature, a solemn and respectful atmosphere came from it. A few minutes later, the Gate opened and a large single eye appeared.
This eye looked eerily similar to the one that appeared every time an alchemist performed Human Transmutation and open the Gate of Truth. The only difference was the fact that this one was golden in color and exuded a breath of mystery.
It was as if all the answers to all existing mysteries could be found from this eye.
The meaning of existence, the origin of magic, the secrets of dimension, the purpose of Gods, and the mystery of the universe. Any question one might want to ask, this eye seemed to know the answer.
After the gate opened and the golden eye appeared, countless golden tentacles that looked like arms pierced Truth's head, attaching themselves to his souls.
The tentacles soon found a large amount of alchemy knowledge from Truth's soul, then a question appeared in its mind:
"Law of Equivalent Exchange. Do you want to exchange these pieces of knowledge for other ones?" asked a cold and indifferent voice.
"Yes," replied Truth humbly.
"Pick your category."
Truth–who was used to this process already–quickly chose a category. Then, an orb was left inside his mind; he knew that this orb contained the knowledge that he wanted.
While Truth was checking the orbs in his mind, the Gate closed itself and everything returned to normal. Truth grinned as he muttered: "This time, I exchange enough knowledge enough for me to study for a few hundred years.
"All my efforts of creating that homunculus were not in vain."
Truth is aware of the need for a large population to develop science and technology. In his case, alchemy. So, he spread alchemy to humans as a way to create more knowledge for him to exchange with the Akashic Record. Basically, he took a similar path as Edward.
However, it was not satisfied as to how things were developing; it was too slow. So, Truth concerted another plan.
It created a homunculus from his dimension with the knowledge of alchemy beyond human alchemists. Then, through a human alchemist of Xerxes, he sent that homunculus to the material world.
That homunculus was the Dwarf in the Flask who would later become Father.
Truth placed the homunculus in an environment where he could observe humans every day, yet could not experience the joy and happiness of being humans. This led to the twisted nature of Father of wanting to become a perfect being, a being better than humans.
Truth be told, Father's entire life was calculated and predicted by Truth. His desire to become "perfect" led to him pushing the boundaries of alchemy to the extreme through cruel experiments–which in turn benefited Truth.
Finally came the confrontation of Father and the Elric brothers. After they defeated the homunculi, the two of them started traveling the world and learning other countries' alchemy, and spreading their own as well.
This led to the rapid development of the field because of the exchange of knowledge and the clash of ideals. Which in turn also benefitted Truth as he managed to acquire more knowledge.
You could say that the entire story of Full Metal Alchemist is a plot of Truth to push the further development of Alchemy forward.
Of course, it's also a game for him. Truth wanted to see whether the homunculus could get rid of his calculation, learn from his mistake, and grow as a species.
It also wanted to see whether human Alchemists could learn the concepts of humility and sacrifice. The entire story is also a game for him to see which of the two species–humans and homunculi–could learn and grow.
At the end, when Edward Elric decided to sacrifice his Gate for his brother, he won the game and proved that humans were the better species. Well, at least, to Truth's standard.
…
Truth started to study the knowledge in his mind, completely ignoring everything else that was going on. As for its clone, it was only in charge of monitoring Edward's manor and nothing else.
If Edward was here, he would immediately understand what was going on: plot armor.
Why is it that Truth would choose this time to have an exchange, making him unavailable to pay attention to anything else? Why is it that his clone was so weak that it could not even notice the spreading virus he released?
It was luck or plot armor. The Luck Potion he drank was working and ensuring that he was a protagonist; everything was working in his favor.
A few days later, the virus had spread to all of the Central Area. Every single individual was infected. Edward stopped and waited for Truth's response. After a month of no reaction, he began the fourth phase of the plan.
Title: Final Confrontation (I)
