The runes add extra gold into play which is needed for this very expensive build. Also since the build already has very large amounts of both mana and AP, the runes give magic penetration. You can use flat MPen. for this build but it really takes off late game where scaling is more efficient.

These are pretty straight forward. AP offensive and mana utilities. The offensive spell boost is for lowering magic resist and chasing anything down.

The first thing you should buy is a tear of the goddess. B back when you can buy it and Ghost on back down. It will begin to charge the Tear/Archangel/Manamune passive. Get boots. You can instead get Ionian Boots Of Lucidity. When and which enchantment you buy is up to you. Turn the Tear into an Archangel Staff and buy another Tear.
SPECIAL ****
What will happen is when you buy the Staff all the mana you earned on the Tear will transfer over to the staff. It will also appear on the new Tear but will not be added to your mana. When you max the mana bonus the Staff will transform into a Seraph's Embrace. The Unique Passive bonus mana will be turned into item stats allowing a new Unique Passive of the same type to take effect. The new Tear will reset it's bonus mana to 0 and begin to recharge. The end result is 3000 bonus mana and 3% of all mana into AP.

After preparing for continuous charging build the Rabadon's Deathcap. It's passive gives a much greater AP boost than another Staff/Embrace. Also, even with champs with rapid fire abilities, like Ezreal, Karthus, and Soraka, it takes 45 mins to charge the 3 in this guide, maybe more.

Prepare the next staff by upgrading the current Tear and buying another. At this point you should have an Embrace, an Staff, a tear, boots, and a Deathcap. Buy the Will of the Ancients and upgrade the final Tear. The Staff and soon to be 3rd Staff should slowly increase in the amount of mana and therefore AP.

What ever champ you use this build on be sure to grab the one with the fastest cooldown first. If they are near equivalent then grab a skill shot or something that doesn't target anything except maybe yourself. It may sound stupid but it is a good idea to fire away at nothing. Remember that each time you use it you get some mana back immediately and increase the amount of damage/effect it will do next time.

I chose these spells because any AP champ can become an extremely dangerous foe. Chasing down a champ with Ghost then, when your cooldowns are ready or they are in better range, slowing them and weakening their defenses and counterattacks can kill almost anyone with minimal injury.

Pros
[*] Huge amount of damage
[*] Huge amount of mana to draw from
[*] Spell Vamp heals large chunks at a time
[*] Continuously increasing power
[*] Big Damn Shield
Cons
[*] Large amount of gold required
[*] Long time to full charge
[*] Nothing to reduce squishyness other than the Big Damn Shield