This is an AP Hecarim guide. Please don't use this in ranked games or people will say "You're trolling, report this dude." If you want to try this at ranked games be sure to practice more before using this build. 2nd guide on mobafire.

Greater Seal of Scaling Ability Powerx9 - Helps you out late game. A great rune for AP Hecarim
Greater Mark of Ability Powerx9 - Helps you out early game and helps your Spirit of Dread to deal more damage early game.
Greater Glyph of Ability Powerx9 - Just like Greater Mark of Ability Power but have more ap on it.
Greater Quintessence of Magic Penetrationx9 - Magic penetration is always good for AP champions.

Kage's Lucky Pick gives you additional gold and ability power.

Sorcerer's Boots give you magic penetration.

Deathfire Grasp gives you cooldown reduction and the ability to
killsteal(don't do this). This is a good item you just do Onslaught of Shadows and then Devastating Charge and you activate Spirit of Dread and Deathfire Grasp he will die quick.

Rod of Agesgives health, mana, and ability power nuff said.

Lich Bane Spam Rampage and attack him every time you Rampage

Rabadon's Deathcapgives more ability power.

Rylai's Crystal Sceptergives slow and health and ability power.

Pros
-Great initiator
-Not that tanky
-Spirit of Dread gives you so much health
-Can do well

Cons
-Ult has HIGH CD
-Really complicated to play
-No damage early game
-AD Hecarim is still better