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.
I play Morgana as a Damage Dealer which has very high utility for the team. With this build you have tons of mana which you of course wouldn't necessarily need. With Archangels Staff you get 3% of your max Mana in Ability Power on top, which isn't displayed here. e.g. 5000 mana would give you 150 Ability Power. (30 AP for every 1000 mana pool)
Frozen Heart should be an optinal item, if the enemy team has heavy auto hitter, the 20% attack speed debuff, and 100 armor will give you good survivability and the 25% cd reduction later in game are really great on morgana, cuz you are totally dependent on your skills.