Viper/Guide

Items
Starting:
 * The and the  early game will give you a boost to your harass and last hitting ability, and can also build into a.

Core:
 * are the boots of choice to augment Viper's DPS and increase his tankiness.
 * gives Viper a decent boost in damage, which helps with last hitting, and can be upgraded to a.
 * is a strong defense item that does a better job than, and the active heal is useful for both yourself and your teammates.

Situational:
 * allows Viper to cast his ult it in teamfights more than once which lowers his target's DPS, slows them, and deals a hefty amount of damage. It also increases his tankiness, and Viper's usefulness goes up greatly the longer he's in a fight.
 * into gives all-around stats which will all benefit to Viper. Dragon Lance alone is like a cheap, allowing to often initiate on a single target from far away, slowing him with the Poison Attack only. Less efficient than the ultimate, however the  active allows to either reach the target faster, or run away from it while still beeing able to hit in dangerous situations. All-in-all, a very good multi-purpose situational item.
 * is another item that provides a mix of damage and tankiness. Once you have a solid HP pool, evasion makes Viper much, much harder to bring down with physical damage.
 * provides an unreliable disable on top of a solid chunk of damage. It's best if your team is low on disables or enemy carries are picking up evasion.
 * improves Viper's damage and gives mana burn. The active can purge enemies and can be cast on himself and allies to remove debuffs.
 * is a good pickup on Viper if you intend to use it for ganking. Viper is a very capable ganker if he can get into close range with a target. Between both of his slows and his damage scaling up as his target's health decreases, Viper is a very good hero for dueling.
 * provides a mix of tankiness, damage, and chasing potential, all which greatly help Viper. It also provides a way to remove certain debuffs.
 * prevents spellcasters from using disabling spells, as well as making your targets more vulnerable to your DPS. The mana regeneration also remedies Viper's constant lack of mana.
 * increases Viper's tankiness, and combined with your and a, Viper will be incredibly hard to put down.