Now, the abilities of upcoming champion has been released. When I search the official forum of League of Legends, I find a
post that the Riot designer explains how he designs the awesome ADC.

Wanna more detailed info? Just check below info out.
Varus' design started with his Q, Piercing Arrow. Right from the start I'd been really keen to try a charged shot, particularly since it's an approach that I've found to be a lot of fun in other games over the years and because it's such a natural conceptual fit with a bow - more time equals bowstring drawn back further equals more range and power. Additionally because of the charge time involved we were able to give the skill a really long range when fully charged, allowing the sort of sniper shots that really capture the sort of legendary archer feeling we wanted with Varus. The other thing I really wanted to try was press/hold/release abilities, and Piercing Arrow was a natural fit for that too. If you're using smart casting, instead of activating the ability once to start the charge and a second time to fire the arrow you'll start the charge when you press the key down and fire when you release the key.
Once Piercing Arrow was working well it was time to look at capturing the rest of Varus' thematics. With his background of being consumed by a dark, supernatural power a mix of physical and magical abilities/AD and AP scaling seemed a natural fit. This also tied in well with a desire to make a wide variety of builds playable if not necessarily competitively viable, something a lot of our older champions especially do really well that I quite like (AP Trist or AP Yi for example). Blighted Quiver was the result of that line of thought, recognizing the magical nature of his attacks and allowing AP to enhance Varus' auto attacks. The Blight stack detonation mechanic then arose out of a desire to amplify the satisfaction/reward of landing Varus' other abilities, all of which are skillshots, and incentivize skilled chaining of abilities and attacks. Damage based off the target's maximum health was chosen to give Varus an edge against tougher front line enemy champions, improving his odds of proccing his passive earlier in a fight.
Speaking of which, Varus' passive, Living Vengeance, evolved hand in had with Blighted Quiver. Given his backstory escape abilities didn't really seem like a natural fit on Varus (he's got no reason to back away from a fight anymore), so a passive that rewards fully committing to a time fight at the right time, potentially allowing you to cut through multiple enemies, seemed like a good match. I initially tried attack damage rather than attack speed, but ran into a couple of issues with that. The first was that it made last hitting minions surprisingly frustrating - a short duration attack boost that's likely to expire around the time you go for a last hit can really throw you off, resulting in minions surviving with a sliver of health. The second was that it didn't fit well with Blighted Quiver and non standard AD carry builds, while attack speed by contrast did.
Hail of Arrows seemed like another natural fit for an archer conceptually - raining multiple arrows down upon one's enemies being a great cinematic sort of thing. The desecrated ground effect went in for both thematic and gameplay reasons. Thematically, since Varus is slowly being consumed by a malicious evil it seemed fitting to have abilities that spread such a dark touch. Mechanically, since he was to have no movement abilities having a slow was pretty important in order to survive/chase as a carry, while a location based healing debuff offered help against sustain heavy match ups that were quite resistant to his Q harass and a form of zone control we hadn't previously explored in LoL.
Finally for Varus' ultimate there were a few goals in mind. We don't generally go for ranged carries with CC/utility ultimates but in Varus' case the lack of escape abilities as mentioned made it a natural fit, in addition to supporting some interesting team compositions with the spreading mechanic for players . An immobilize worked well with the rest of his kit, allowing him to charge up Q shots and/or set up a target with Blight in order to proc his passive. Thematically it was also a great opportunity to really show off the power of the corruption Varus had turned to to get the power he felt he needed to exact revenge.
I think you may be like it------
Upcoming Varus Classic Skin & Sounds Teaser in League of Legends