Mordekaiser is often built as a tank, which isn't bad because his passive makes him one of the most difficult tanks to deal with. But being nearly indestructible just won't cut it in the team fight; add some AP buffs and you've got a force to be reckoned with.

Max out E first, Siphon is simply his best attack for a variety of reasons. The range of the attack is impressive, it practically refills your shield bar, it's the perfect farming skill, the cooldown time is very short, and with enough AP it deals a hefty amount of damage to all targets in the cone.

W is a built-in Sunfire on roids. It boosts armor and Mres, and deals damage per second to all surrounding enemies. Also you can protect an ally instead of Morde, so if your Tryndamere is diving into their team to turn the tide, you can hit him with the W to make him that much more difficult to deal with. A very good, very flexible skill.

Q is a great skill also, but for this build Mace of Spades is not needed early on. However it is extremely useful in the later stages of the game,