Starting with a mana crystal isn't my thing soul stealer isn't good on a champ who normally dies to kill

Nice build but I think it's (mostly) copied from the original version of that top-rated build with very similar name. It's not word-to-word copy but I can't see any important difference except explanations which are quite poor here (and very good in that top-rated build).
I can't downvote because I agree with this way of playing Karthus BUT I can't upvote because I consider this as plagiarism.

Do you think it's better now ? Description is poor because i don't know what to say and things like "hi i am really happy i created this build blablabla are boring and useless"

Disliking how you buy Boots first.

Your description is also very poor.
A+ effort for the Tips section, but the rest of your description seems like filler.

Everyone knows what your skills do, and you don't really explain anything on the item build.

Thanks.

Nice build !