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.
Why would anyone make such thing as a Malphite-mage? Well, Seismic Shard and Unstopable Force depends on AP and scale pretty good with it.
Reazons for the item choice? Sorc Boots - more magic pen, Rod of Ages - life, mana and AP, Rylai's Scepter - you don't want them to flee after you crush in their party with Unstopable Force and Slam the Ground, or don't want them to be fast on your heels when everything turns bad, Lich Bane - the only item wich would help you tresh some towers, and do some phisical damage to, Zhonya's Ring need no eggsplenation, Thornmail - you wouldn't expect this on a mage, do you? It improuve your Ground Slam, besides more armor and - when a dd hero hit you he hurts himself good, especialy considering your high magic pen.
Other good items you may use - Sunfire Cape, it's aura will only benefit greater from your magic pen, also give armor and bonus damage for GS; Abysall Scepter - goodie, even more mag pen!; Guardian Angel - it give you a second chance, sometimes (other times the enemy just sit near you till you live (a short life) again.
A good combo: Unstopable Force, hit (with Litch Bane + 150 AP to damage), Ground Slam, hit again (with same bonus), Seismic Shard then run (one way or another).