Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
As you can see I add AS and some AD to Mord unlike most (wow you can help kill towers now). This is key and makes the difference. I have spent the most time on Mord compared to any other toon and this build works better about 90% of the time. If pure tank is what you need then choose another toon. On a team you should have another hard target that compliments Mord like Galio, Amumu, Nunu, Shen, etc...
Although you are the master of metal you can't push them away from creeps until level 2. So relax and last hit those creeps till you get your mace attack using syphon. Once obtained try to stay on the back side of their creeps forcing them away while taking last hit for gold on the creeps. This will control your lane and make your opponent frustrated and p/o'd that he can't do much (watch for ganks because you are very annoying, use wards and ward the map for your team mid & late game). You will be earning the most gold on your team if you play correctly and should enjoy this task because your team will die less and that is good for you.
My favorite. At this point you should have Stinger, Boots (by the way, if they have a ton of stun, silence, etc... get treads please). You should be pushing your lane and running to drop wards now. This will allow you and your team the ability to converge on people in the woods trying to gank and farm. When I push a lane back, I run to the nearest minions in the woods and grab some free gold. I push the lane back again and repeat. Playing mid is my fav because there are wolfs on one side and ghosts on the other and I can push, take minions on left, push, take minions on the right. I will out gear the other player and crush them when ult and ignite are up and I lower their health a little before fully engaging. Get fed and don't die by chosing target you know you can kill and will help you kill with it's soul like Twitch, Yi, etc..
NOTE: If you are playing against an invisible toon, get oracle early. Just do it.
By now you should have shown the other team what you are about and they should be scared. You are a tank of sorts and yet do tons of magical and your share of physical damage. Keep the lanes pushed for your team and don't die doing it. This requires being smart and not pushing too far when you can't see the other team. Pay attention, remember those wards? Keep em coming. Full team fight you should take out a carry with your team and steal it's soul. Now if you killed fast, the fight is 6v4 and Mord scores penta kills with those odds.
Note: I put thornmale in the list only if you are playing a dps like ww or someone like him who you will wreck if you wear one, otherwise leave it. If you are against a predominantly physical team get Randuin's Omen. Learn to use your item abilities from these goodies and you will have 1 or 2 extra skills.
Keep your shield up as much as you can
Takes tons of abuse
Outputs tons of damage
Pushes lanes like he's on steroids
Steals souls and turns the odds in a fight
Stun, silence prevent you from maintaining shield
Can be squishy under the shield
If you die the soul dies to
Skill sequence may be off slightly but lvl mace before syphone and save your creeping death to level last.