How to Customize the End Poem After Beating the Ender Dragon
Find the End Poem File
The End poem text lives in the same ./config/customcredits/ folder the mod creates on first launch. Look for the file that contains the default poetic text shown after the dragon dies. Open it in a plain text editor. The poem is displayed line by line, so each line you write becomes a separate line on screen.
Write Your Themed Text
Replace the vanilla poem with your own story, lore, or ending message. This is popular for themed modpacks where the dragon fight is the finale of a narrative. You can reference your pack's characters, tease a sequel, or write a short in-universe farewell. Keep lines reasonably short so they fit the screen width, and leave blank lines where you want pauses in the scroll.
Test and Iterate
Defeat the dragon again (or load a world where the credits can be triggered) to see your poem in context. Because the mod reloads the file when the credits screen opens, you can edit, save, and re-trigger without restarting the game. If you want to check pacing, watch how the lines scroll and adjust blank lines or line length accordingly. Save a backup copy of the original file first in case you want to restore the default poem later.