What does the "serious damage" mean exactly? It doesn't make much sense since this is not a stats game.
Let me get this straight btw. Ulli you had the diary, gave it to Klippy in secret, then someone stole it from Klippy, after you "derped". And now you claim you were given this truth potion.
So, according to this post
"The Diary's magic was affecting him, surely. Even though their motives aligned even he was not immune to the corruption of the Diary."
(too lazy to quote) does You-know-who have the diary now?