Eren's words were akin to a spell.
The immortal magic reached its peak when he expressed his feelings through his actions. And it started working when he verbalized what he felt at the time.
Reen's unstable form showed signs of change. Her Soul Seed started spreading its spiritual roots once again with a newfound vigor right after Eren's heart vanished into the vessel.
Skreeeeeeeee!
The world of Anfang was shaken once again as the soul fragment's alter ego took control of Reen's vessel. When Reen opened her eyes, they exuded a divine might that Eren was not personally familiar with. Yet, he could sense a familiarity coming from it.
A sky-piercing manifestation of a cosmic entity was formed above the sky of the battlefield. This manifestation was so big that it could not be fully seen by Eren and others around him completely.