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.
CONCEPT: Adapting new mord to fulfill his intended damage role while still keeping him tanky. I find this build has an easier time snowballing with mejai's due to his increase in burst potential from runes as he levels and his ability to easily maintain stacks with all the tanky stats.
PREFERENCES: Swap doran's shield for a regrowth pendant if you please. Both are fine. Obviously adapt your item build to your enemy and prioritize armor or magic resist accordingly. Th last item slot is flexible. I favor rylai's for the additional slow on top of the health/ap. Other strong choices include Zhonya's, Lich Bane, Hextech Gunblade, Void Staff (to add to your already strong MP), or possibly another tank item. The core build is a bit lacking in magic resist and can be slow when building up defense. Judge whether it is appropriate to prioritize damage, resists, or health as you progress.