Eren was becoming a summoner.
This was not what he had signed up for when he had decided to join the Titus competition. He never would have imagined changing his class at this point in his life. If he had any control over the process, he would have aborted it long ago.
But for better or worse, he didn't have any control while he was in that state. He felt like he was being played by his bad luck again.
Eren could understand everything that was happening to him. But he was a prisoner inside his own body with no control over its functions anymore.
Things only got more difficult for Eren from that point. He was then gifted with a killer headache.
It was as if someone was drilling through his crown to take his brain out. Eren was in so much pain that he couldn't shout or yell anymore. All he could do was bear the uncalled-for pain and wait for it to end.
And yet... things weren't as terrible as he made them out to be.