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.
Since Mord's abilities require a bit of your health, Regrowth Pendant is a good starter. For spells, take Siphon of Death, then Mace of Spades, Siphon, Mace, Siphon, Ulti, and so on... Creeping Death take the last. Boots of Swiftness gives you most movement speed, so you can chase down most opponents. Phage, later Frozen Mallet, slows enemies on hit, so getting them is sure. Malady gives you big attack speed boost, and life steal. If you face a good tank with lots of health, cast Siphon, then Mace, followed by Children. If you have Ignite, cast it too... Then Creeping Death, for resistances and dps. Warmong's armor requires Regrowth Pendant, so you have one more slot for Infinity Edge. At most games, when I could get all the items, i was almost unstoppable.
Good luck. ;)