Get for more gold & sustain
Mejai Head Start...
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 Early Zz'Rot?
Building an early Zz'rot allows for a lot of map pressure and potentially early tower gains. The idea is to place your Zz'rot behind your tier 1 tower so that it cannot be killed. The subsequent mejai, which is already good on Janna, gives her the AP she needs. It's already harder to kill her so it's smart to build that next.
The sightstone can be added earlier if needed, you should get by on pink wards with your teammates and mute teammates that complain.
Feel free to follow up with a Banner of Command as well.