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Mordekaiser benefits from many things in many different mastery trees, though obviously, avoid putting points in mana regeneration, as he has no mana to regenerate. Mordekaiser's strength comes from staying alive long enough to charge his shield, so no matter how you choose to build him, it is a good idea to put some, if not most of your points in the defensive tree. Ardor is an excellent talent on Mordekasier, as is Veteran's Scars. Like other heavy melee champions - such as Nasus and
Malphite - even an offensive build benefits from a defensive spec.
Somewhat counter- intuitive is the fact that Mordekaiser benefits greatly from cooldown reduction. In addition to making Children of the Grave usable more often, CDR allows Mordekaiser to use his other abilities more frequently, thus keeping his shield charged. This can make going deep into the Utility tree a viable decision, especially since it includes Quickness, which boosts run speed. Mordekaiser has no slows or disables, so he needs all the speed he can get. Because of his great farming ability, the XP boost talents can be a good choice as well.
Likewise, picking up the crit, attack speed, CDR and armor penetration talents in the offense tree makes a degree of sense as well, especially if you are building Mordekaiser as a heavy melee. Regardless of your choice, it is recommended that you go nine points into defense and pick up Nimbleness - this talent gives the champion a run speed boosts when an attack is dodged. This is an excellent skill for Mordekaiser since he'll be deep in the fray no matter how you build him.