Path out of the Valley
Huddled together, the large number of illusory beasts seemed to have undergone some kind of metamorphosis as they seemed to momentarily merge together for short periods of time to launch attacks towards the barrier. From time to time, some of the illusory beasts would be too exhausted to merge together and would separate from the mass and leave to recover while another illusory beast took its place to keep up the attack on the barrier.
"…"
To describe it in more simple terms, all of the illusory beasts became a single large illusory beast for a short period of time and used this giant form to attack the barrier. Repeating this process over what should be at least a year at this point in time, the barrier that kept them from leaving the valley had weakened to the degree that it looked like it was about to break at any moment.