Mordekaiser is a tanky, high damage champion who offers low mobility, but can easily destroy high priority targets like adc with the use of his ultimate.

Mordekaiser is actually a highly versatile champion that can build practically anything as long as you build a Rylai's Crystal Scepter. Most of his damage comes from his passive, so even if you don't hit your Q, you can still kill your targets fairly easy.

Bramble Vest is best used against champions like darius, and renekton. they can sustain off their skills and conqueror, but getting it early guarantees they cant sustain of of you when they are fighting you. it is cheaper than a morellonomicon, so it is a high quality item on mordekaiser.

Trinity Force is a really good item on mordekaiser, it grants him atk speed to deal more dps since his passive also gives on-hit, and gives you hybrid damage if your enemy decides to build MR. it also gives you 20% cdr which is definitely very useful for mordekaiser.

Rylai's Crystal Scepter gives you permaslow with your passive. since your passive gives you movement speed, this item allows you to absolutely guarantee being able to chase your enemy down and kill him. it gives you hp and alot of AP, overall cost-effective item on mordekaiser since it's a core item.

Hextech Rocketbelt gives you a little bit of mobility and waveclear, also gives you hp and cdr, overall cost-effective item on mordekaiser. You may not even need to build this.

Dark Seal helps you ballin', you dont even need to upgrade this to a mejai's because it's a waste of 1050 gold upgrading, but if u have nothing else to build, upgrade it to mejai's.