You should use this build because it gives you a good early game with the aftershock. You start off with your q and when you land it you get a HECK ton of armour then you can harass your opponent in that period of time. The rod of ages is a great item for cho'gath, has almost everything he needs. After buying the ROA you can get your Boots and after you can rush Righteous Glory or abyssal mask. Then you have two slots remaining for more AP go for zhonya's hourglass and banshee's veil. For defence against AD frozen heart is a must have, dead mans plate helps you with speed and a little more dmg. Against AP spirit visage is a good item to get, or adaptive helm against spell spammers like cassiopeia. Against champs with life steal for example Warwick, thorn mail will destroy their chance of healing. If your fed Go gargoyle stone plate or go zz'rot's portal if need to destroy towers even quicker. Other items you can get are warmers for the health and region, Liyandry's torment for MP and AP, Ohmwreker for tower dives and armour, and Seraphs embrace for.... ummm IDK. Have fun!