Hello everybody,
My item choices:
This 3 items i nearly get every game. You need the healthregen of FoN also if they have only 1 ap. Sunfire cape is also a good choice on morde. it gives health, armor and its passive fits perfectly with your creeping death. rylais scepter for more survive ability, slow and increased dmg. The last 2 items depends on the enemy team. a lot of ap dmg? -->abyssal scepter. a lot of physical dmg? --> thornmail. ofc you can get both items for last choices when there are a lot of both dmg types. If its not so the 6th item depends on your play style.
more tanky after abyssal scepter?-->Warmogs, cause you have a lot of magic resist with FoN and abyssal scepter. If there is a ot of cc you can also get Banshees.
More tanky after thornmail?-->Warmogs/Randuins or for atk speed stacker like xin or yi Frozen heart. The cdr is also nice.
If everything is good and you dont die very often go for rabadons deathcape or Void staff if they have a lot of magic resist.

Farming with morde is very easy cause you have a lot of aoe and your sunfirecape.
play a bit carefully without FoN and sunfire cape and remember in early game that your skills cost life. You can with morde easily initiate fights if your team dont have a "real" tank. But Amumu, shen or something like that is good to have on your team, if not your team should not be very squishy, another off tank like olaf/garen is helpfull. Dont initiate with your ult, use it when the FOCUSED enemy has less than half of the life and you nearly sure that he dies. The Ghost-Pet you get is very usefull for pvp and pushing.

For Summoner spells i choose ghost and ignite. ignite fits perfectly with your ult and ghost is nice to chase and escape. You can choose also flash for get out of teamfights with low life for example. ghost helps me more to chase. You can easiely tower dive an enemy (without range stun) in early/mid game. run with ghost in ignite,ult if nedded e skill and run out. same with flash, decide on your own if you want flash in or out.

With this build i had a pentakill and every game positive stats. This is my first guide, excuse me for the unformal text ;P