Powerful harassing tool. You can use this ability to harass your opponent without dragging the creep aggro on yourself. Aim to get 5 stacks of Poison Attack on your opponent to deal the maximum amount of damage to him.
Nethertoxin is one of very few sources to disable passive effect of enemy heroes. This spell is also great for farming and split pushing. With the help of level 25 talent Nethertoxin becomes one of the strongest abilities in the game.
Corossive Skin is very dangerous in the early and mid stages of the game. This spell makes it a lot harder to harass Viper.
Viper Stike has a low cooldown, so do not be afraid to use it often. This ability applies insane slow and attack speed de-buffs, while also dealing quite a lot of damage.
In combination with range talent, Dragon Lance allows Viper to hit his opponents from a very large distance.
Get MKB exclusively if you play against heroes, who have any powerful source of evasion.
Viper is not very mobile, which makes it easy for enemy heroes to get to him in fights. Black King Bar is going to allow you to feel a lot more comfortable in fights.
Hurricane Pike helps Viper to improve his poor mobility.
Butterfly is a great item for an ability ranged hero. It gives a lot of armor, attack speed, movement speed, evasion and damage.
Satanic is one of the strongest items for core heroes in the late stage of the game. This item significantly improves survivability of his owner with the help of its active ability and status resistance bonus.
Decent item to get in case you play with a hero, who has a powerful passive. This item also suits players with active playstyle.
Great alternative to Satanic or Butterfly. Additional stats and passive slows will strenghten your hero.
HH is a powerful counter to cores, who can deal a lot of physical damage.
Manta Style is a decent alternative to Black King Bar, in case the enemy team doesn't have a lot of disables.
As Viper lacks disables, it might be necessary to get Scythe in the late-game to gain the ability to catch mobile enemy heroes.
Viper in current build is pure physical damage dealer. Daedalus will significantly improve you damage output.
Great item for farming, fighting and split-pushing.