Nightmare Fuel / Once More with Feeling

• Due to Instrumentality, Shinji remembers too well how Asuka died... and he relives that moment in his mind over and over. It has become carved into his brain.
• In-Universe example: The memory of Asuka fighting against Sandalphon inside the volcano and sinking to her death in the depths of the magma gives Shinji frequent nightmares.

---