This is just a troll team I came up with that has a simple goal: don't let the Karthus die. EVER.

Note: THIS IS A TROLL TEAM. IT IS NOT MEANT TO BE USED IF YOU WANT TO WIN. Not saying it isn't possible, but i doubt it is actually viable.

I can't help but feel I'm missing someone who would fill the last team slot better than Soraka. Let me know if I'm right about that.

Pros
Karthus has great damage output, and will NEVER DIE.
You are tolling people, so the rage may be hilarious.

Cons
Karthus may die at some point
You may get trolled by this, and they may find the rage hilarious.

Karthus: Build lots of AP. And don't die. You job is to do as much damage as possible, their job is to keep you alive. You only defensive item is the Guardian Angel, just because it fits way too well with the troll nature of this team to be left out.

Kayle: As soon as Karthus's Guardian Angel is triggered, use your ultimate to make him invulnerable, giving the rest of the team time to save him. Most of your items are designed around move speed. You will be moving VERY fast.

Zilean: As soon as Kayle ults Karthus, you should use your ultimate on him. You build more AP than anyone (except Karthus) to maximize the effectiveness of your ultimate.

Yorick: If you see Karthus drop after Zilean ults him, it's your turn. If you want an explanation as to your build, I'm not realy sure. Most of what I know about Yorick is that his ultimate fits into this team. So I gave you aura items, because why not?

Soraka: Any time you see Karthus low on health/mana, fix it. Your build is basically all aura items, so stay close.

There are only 2 mastery trees here, and they're very similar.

Karthus gets spell vamp and move speed instead of gold/10 and bonus xp. Because he's Karthus.

Explanations for everything else

These masteries maximize CDR, allowing everyone to get their ultimates more often (everyone but Karthus should have the 40% CDR cap at full build).

Since you have to invest so heavily into the Utility tree to get the CDR from it, I figured why not go ahead and grab Mastermind , since Revive has such a long cooldown.

Good Hands is taken to get the whole team back on their feet and back to supporting Karthus

For Karthus, these are max damage runes. And Revival. In case Revive is on cooldown and your team lets you down, it minimizes the amount of time you have to spend raging in all chat about these noobs (in jest, of course). Raging is hard work after all.

For everyone else, you want defence. You need to survive to save Karthus. Hence, everything is /level. And the CDR brings you to the 40% cap.

Karthus: Lots of AP, MPen, and a GA. All you really need.

Kayle: CDR boots, GA, FoN (for survival/move speed), and a lot of PDs. Because PDs are awesome on Kayle.

Zilean: CDR boots, GA, FoN, and AP, to make full use of the scaling on your ultimate.

Yorick: CDR boots, GA, FoN, Triforce (everyone loves Trifoce, right?), Aegis (someone has to have it), and Locket of the Iron Solari (ran out of room on Soraka)

Soraka: BoM (CDR from Herald), GA, FoN, and aura items. Because you're not playing AP Soraka.

Level your ultimates where possible, and for Kayle/Soraka, level your heals.

Honestly, aside from those rules, feel free to skill whatever you want. I just guessed from hearsay about what abilities did (especially for Yorick, who I've never been in a game with).

Revive and Teleport. No alternatives.

Revive: If you're dead, you're not saving Karthus.

Teleport: If you're in base, you're not saving Karthus.

Combo should go as follows:

Engage
Yorick activates Locket of the Iron Solari while Kayle and Soraka heal Karthus, who should be dropping abilities all over.
When Karthus goes down, he is revived by Guardian Angel, and then Kayle targets him with Intervention, making him invulnerable.
Zilean Chronoshifts Karthus, reviving him as soon as he dies.
Yorick ults Karthus, allowing the lich to get in double the final damage.
Team gets aced.
Team Revives.
Team does this again.

I feel this is most effective with Karthus mid, Yorick/ Kayle top, and Zilean/ Soraka bot. Because of Zilean's passive and the XP gain masteries, everyone should be well ahead in levels, despite no jungler.

The reason for no jungler is that there is no room for Smite in this team comp.

I hope you have enjoyed reading my guide to the Immortal Karthus troll team. Give it a try.

Let me know if there is a better 5th team member, and what Yorick should actually be building.

Go forth, and troll.