Mostly AD Varus
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.
How could this work?
The theory is simple. Varus naturally comes with hybrid damage in his kit. This gives you the opportunity to adapt to the enemy team and their defensive build to make sure you don't fall off in the late game.
If you have an AP heavy team on your side, the enemies should be building lots of MR, so you can grab Whit's End and Runaan's Hurricane to help shred for your team.
Conversely, if there's a lot of armor slowing you down, grab a Last Whisper and Static Shiv.
Blade of the Ruined King serves two potential purposes: You get wrecked by an assassin and you need an extra heal to survive a burst OR you need help chasing down some seriously slippery targets.
Those masteries are whack.
"And maxing W is for AP Varus. wtf are you doing?"
There are two really powerful masteries in the cunning tree: Precision and Thunderlord's Decree.
Precision provides balance piercing, which is especially helpful on Varus who has mixed damage.
Thunderlord's Decree is a huge harassment tool. With your abilities, you can easily pull off the three quick hits necessary to trigger it.
Which brings me to the most important part of this guide...
Most ridiculous harass on the Rift.
Quick tip: Getting 3 Blight stacks triggered is great, but you really don't need to hit them every time.
In lane: get two auto attacks for the stacks. Immediately fire a short-range Q or E. This gives you 4-10% of their life from Blighted Quiver AND triggers Thunderlord's Decree for even more damage.
Rinse and repeat every 20 seconds to take off chunks of your opponent's HP and send them home to heal.