Riftmaker is a core item; Liandry's and Rylai's combo is situationally strong. For tanks/bruisers, prioritize Riftmaker-Demonic Embrace-Liandry's; for ranged, consider Cosmic Drive or Rocketbelt.
Mordekaiser excels as a counter-pick but struggles as a first pick against counters like Olaf, K'Sante, Jayce, or Gnar. Banning them is advisable, and consider Ignite for tough melee matchups.
Mordekaiser is powerful after level 6, so play aggressively then. Essential secondary runes like Revitalize and Bone Plating significantly boost your survivability and combat sustain.