This is Nefarius's Tutorial ... it's not a specific level 500 tutorial, but the info within it explains how you could create your level 500 ... this is the one and only tutorial by Nefarius on this subject 
HIGHER LEVEL TUTORIAL
BY NEFARIUS NEFARIUSDX@HOTMAIL.COM
IN THIS TUTORIAL YOU WILL LEARN HOW TO BREAK THE LIMIT FOR MAXIMUM CLVL
AND SET IT FROM 99 TO ABOUT 120.
1. FILES YOU WILL NEED:
CharStats.txt - you actually dont need to edit this, but it would be good to adjust the stats gained per level.
Experience.txt - this is the main file we will edit.
2. Open up Experience.txt. now you will see alot of numbers, these are the numbers of exp you will need from clvl0
to clvl99, now what we want is to be able to get to clvl120 right? first lets set the limit in the MaxLevel row (row 2 in excel) change the entry from 99 to 120, as you can see you can do this individually for every char but i dont think it would be nice if only one class can reach clvl120 now would it? but leave these alone for now after you set the MaxLevel to 120 instead of 99 you will have to add the new level rows, when you scroll down to the bottom of the .txt file which is at row 102 and the name of the row is 99 in the same column continue the list, below 99 type 100 below that 101 and so on until you reach 120. now we have set it up to 120 but the exp columns are still empty so what we can do about this are a few things, the max exp in Diablo II is 5,000,000,000, this can be changed but we are not after making things more complicated. there are two things we can do about this, one is to delete all the exp data
from lvl1-99 and type it from scratch, or we can do it in a faster way, the exp gets almost ungainable after level 85 when you start needing tons of exp for a single level up, so delete all entrys after level 85 (row 87) and re type them so that at row 122 (lvl 120) it will reach 5,000,000,000 or less, note: make sure you dont have levels which need more exp to reach then the next level (i.e. you need 2,000,000 exp to get to lvl90 and when you have 2,000,000 when you reach level 91) this will make diablo crash since your exp is below the exp needed to reach the level you currently have (it will not crash if you have a level that is lower then you exp in that case it will bump you up to the matching level)
3. put the file in your modding sub directory or put it in you patch_d2.mpq and test it, how to test it you ask, if you have chars that are above level85 and the exp they need to reach lvl86+ is the exp they currently have then when you kill something like i said before you will be moved to the matching level, but make sure you dont have a char that has less exp then his level, since in this case the game will always crash.
4. if you put the limit to about 200, 300 or even 900 then you will need to edit CharStats.txt for one simple reason, you dont want 5000 base hp chars to run arround your mod, you will have to balance the stat gains per level so that they dont get inbalanced when above level99, the best way is to decrease the life per level by half, take as a note that these values are all :4, which means that 4 hp per level will be marked as 16, 2 hp as 8 and so on. do the same for mana and stamina and you should be fine.
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"To dream a life, or to live a dream ... only through desire shall the sleeper awaken."