Nice improvements :P
I gave you a +1 for the effort.

You could learn some coding in Jhoihoi's how to make a guide :)
I don't have the link now, but you can find it by writing karthus guide in search :P

Hey dude, i liked your build, i would just suggest don't even getting the Yommus, if you just sell your brutalizer and get Last Whisper some gold can be saved, however, the Yoummu's active, and cooldown reduction part can be of use. Also, never tried Wit's end before, seems to be a nice idea, next time Renek gets on free week i will try it, just suggesting a Bloodthirster on late game, you will be pretty much a dude who can triple kill their squishies while tanking them all, pretty cool imo. +1 =D

Sunfire cape isn't that good of an item full stop and its even worse to rush since you have no magic pen it doesent do much damage and randuin's is better for defense.And attack speed isn't the best on renekton

Hello :P
Just few things I'd like to point out, other than that your guide seems good :)

*You should get those boots earlier, I recommend Ionian boots instead of mercury treads
*The Brutalizer and youmuu's ghostblade really early, sunfire cape is quite hard to pull off as first item, because it's snowball champion
*Not so sure about Wit's End, the attack speed isn't really needed. You get that from possible youmuu's ghost blade.
*Desolation marks ? I guess they're fine also, but flat attack damage for snowball champion is better
*Slice and dce at lvl 4, not 3?

Oh btw, instead of putting pictures of garen for example, use this [ icon = garen size = 50 ] (Without spaces, except on the garen and size) it will give you the image and link to it.

Keep on working the guide, because it has potential :)
I won't upvote or downvote, but I can check the guide again later on, if u wish :P