Mordekaiser should always buy Health regan first, since he needs his health to cast ability. Also, mordekaiser is a ap champion. So you should buy him some spell vamp to give him some damage. Mordekaiser requires few Ability power to do a high amount of damage. So after buying boots and blasting staff, Mordekaiser's health is good enough to go close range with people. So now we need some Hp to keep him in battle all the time. After Warmog's armor, he is a tanky ap champion. Afterwards, we can him some AP items to get him damage to kill during battle. This build actually help me win battles in ranked game. Hope it works for u guys =)