Is an absolute must, all of the stats are important to Karthus early/mid game. Next you'll want to grab
. With your runes you will be up to 26 Mpen almost completely negating the base 30 ranged champs have. That's not even counting the Mpen mastery which should reduce them to 1-2 magic armor.
After Roa I like to immediately go for a
and eventually turning it into
Start working on this when at least 2 enemy champs have 70+ MR. If for some strange reason none of them are building MR, don't bother with this. In fact I'm pretty sure the game is going to be over soon since you'll be nearly one shotting the glass cannon enemy team.
All around good stats even excluding the active. I consider it a must have because you will be diving team fights by flashing behind the enemy and walling them in.
These two are important to switch up based on the enemy team. If you're taking too much magic dmg and want even more mpen grab that Abyssal. Is someone like Jax/Trynd/Rengar always diving you? Grab Rylais. Not saying the slow will make a huge difference against Rengar but the extra hp could be just enough for you to get into position before dying. Also with Rylais you're giving off a constant -15% slow to everyone which is always useful. Not to mention all your single target Q's will be doing the max -35%.
I almost never get this which seems to have alot of Karthus players questioning me. It's only 700g for the tear blah blah blah. You have to understand that rushing the sorc/abaddons at mid game is VERY important. Practically everyone I see making guides or talking about Karthus says his job is just to afk farm. This is only true for the lane phase before you have
. After you get those two you want to "afk farm" until your ulti is up and then either A: Gank, or B: Force a confrontation. I suggest B if possible, have your team start dragon and just wait behind for the enemy to show up.
It's also worth mentioning that Liandries is a very good choice as well. When the enemy is walled and stuck inside your E you will be damaging them all for a high static amount. In fact it might be a better choice than Abyssal. I've just always had a use for the MR on Abyssal. If you think you can get away with base MR values then go ahead. Plus once you have 6 items you can always just sell your boots and grab an Abyssal then.
Lastly if you are feeling like you're on the top of your game and want to make the early game as painful as humanly possible for the enemy, rush Rabadons instead of RoA. A first item Rabadons coupled with Sorc boots is devastating if you don't get one shot out of position before laying down the pain. Void will be a better choice if most of the enemy team already has 50+ MR but that's unlikely unless the support is Galio and mid is Vlad.