When Mordekaiser's health falls below the cost necessary to use a skill, his health will drain to 1 HP. Any subsequent spell casts will bring him down to 1 HP, meaning that he cannot kill himself through his own abilities.
It is often a good idea to farm conservatively early game, as your Iron Man shield is not nearly as effective as it is late game.
Iron Man is a great late game ability allowing you to survive during team fights longer than most other champions, as long as you use your abilities constantly.
The damage from a level 2 Creeping Death is a bit greater than the damage from Sunfire Cape but Sunfire Cape still gives more armor than level 5 Creeping Death. This makes it very effective at farming minions, and during fights. The bonus armor that it gives can also help tanking turrets. You can use it on an ally while staying in the back of the fight at low health to build up your shield and get assists.
Leveling up Siphon of Destruction first gives you the ability to poke, but mind its health cost, as it can quickly bring you down below a comfortable amount.
Creeping Death is usually the best ability to gain shield health, due to its consistent AoE damage.
If you use Children of the Grave on their attack damage carry and take down the champion quickly, the ghost's auto attacks will fill your shield up and help you stay alive longer.
Like most other secondary units, the soul you've stolen with Children of the Grave can be controlled using Alt + Right Click. However, unlike most secondary units, your ghost acts as an allied champion, and will be affected by abilities that can only target champions, such as Wish, Requiem, and Infinite DuressInfinite Duress.
Mordekaiser's ghost will attract tower aggro first, but if it attacks a champion while you are in range, you will become the new target. If you cast Creeping Death on the ghost and it hurts an enemy champion, you will draw aggro instead of the ghost.
Mordekaiser is very capable of taking a solo lane against 2 enemy champions, reaching a higher level than both of them while effectively harassing and farming at the same time. If you harass them enough, you can potentially get a double kill with your ultimate.
Take advantage of the fact that many people underestimate Mordekaiser's damage once he gets his ultimate, when combined with Ignite and his other abilities he can kill enemies who feel safe if he can get in range of them.
Getting magic penetration runes, and Sorcerer's Shoes allows you to harass better, get more kills early in the game, and even late game against those champions who do not have any magic resist above 30. The full power of your ultimate can be extremely devastating to squishies.
Magic penetration usually is more effective on Mordekaiser than flat ability power, as the AP ratios on his abilities are relatively low. Building magic penetration makes your abilities hit much harder, giving you even more shield and letting you stay in the fight for longer without sacrificing damage. It is often more effective than stacking health in most situations.
The pet that Mordekaiser gets from Children of the Grave is considered a champion. It can not be affected by abilities that target only minions. That also means all abilities that target only champions will work on the pet as well.