Ok, someone explain this to me.
WHAT THE FUCK HAPPENED THERE?!
Eren is reliving memories of past events that never happened to him. In this page, he is recalling arriving at a chapel at night, entering the chapel and finding it empty, and opening hatch leading to a secret underground passage. The last panel is him descending a dark stairway.
It is assumed that he is seeing all this through his father's eyes, as it is implied that these are Grisha's memories.