When should I run each build?
Run Lethality when you need to play from a fair distance due to poke/assassins or the enemy team has 95% squishy champions, Run AP when the enemy team has big HP stackers that you cant walk up to, and run on-hit when you have peel and can get closer than usual to deal damage in teamfights.

why Prowlers?
Although you don't use the active dash often, the stats that Prowlers provides from not only the item itself but also the Mythic passive outweigh that of the other Lethality Mythics. You get significantly more Lethality than Eclipse with the same CDR as Duskblade, but per Legendary you get 5+ Lethality making your damage noticebly better than both Duskblade and Eclipse. The Mythic dash isn't even terrible on Varus, as if you are getting dove you can use it as an extra outplay tool or even a finisher on a low health target with Flash + Prowlers + WQ.

Which build is the best build?
They all serve their different purposes and are all uniquely useful in different scenarios. Regardless of this fact, you can pretty much go whatever build you want every game consistently if you really wanted to and still have a good winrate as long as you play it correctly until Master+.

Why AP Though? I thought Varus was an ADC.
Varus has extremely surprising AP scalings in his kit, with his W's max HP% damage scaling with AP (2.5% per 100AP) being the most potent allowing him to one-shot almost any target lategame, especially anyone that stacks health.

Will you update this guide with combos and more written advice?
Eventually, hopefully in the coming weeks i'll make it more expansive and tidy it up a bit.

Any other questions/Queries? Contact me on Discord! Shoppe#9292

Try to aim to use your E to hit the first 3 CS in lane and poke the enemy at the same time

Q Damage increases with charge time as well as damage done per Blight stack

Your Q has 2.4x~ Caitlyns auto range for scale, making it an extremely potent poke tool especially with the Lethality build.

Your W Active provides extremely potent missing health damage, make sure to use it when the target is on lower health rather than higher health.

Your E applies grievous wounds and slows in a circle for 4 seconds, use it both to proc your blight stacks and in a teamfight as utility.

Your ult doesn't immedietly apply all the stacks, so charge your arrow for a second and a half before firing. Especially important for getting the maximum damage on AP Varus.

Varus has a strong lane phase provided you pop your blight stacks correctly, so make good use of it.

Use your Ultimate pretty much whenever it is up, as the cooldown is quite fast and you can usually catch people off-guard with a quick burst from the blight stacks it applies and the root.

Any other questions/Queries? Contact me on Discord! Shoppe#9292