Tips to Use Nyx Assasin (Dota2)

• Impale's effective range is longer than its casting range. It can be necessary to target the spell on the ground to hit targets just out of normal reach.

• As Vendetta's invisibility dispels only when your attack connects the enemy, it is nearly always best to deal your backstab damage before your Impale and Mana Burn follow-up.

• Use Spiked Carapace only just before your Impale's stun times out, so as to reduce the enemy's chances of noticing the effect and holding back their counterattacks until the Carapace expires.

• If you know that you are targeted for a delayed or long-range nuke (such as Sniper's Assassinate or Invoker's Sun Strike), it can be a good idea to purposefully time your Spiked Carapace to reflect the incoming damage. This will aid your team by disabling or even killing the caster. Spiked Carapace makes Nyx a good counter for Keeper of the Light through the early-mid game.

• Vendetta's fade time into invisibility is instant, and can be used to dodge many spells provided the enemy does not possess True Sight.

• It can sometimes be necessary to not use Vendetta to open ganks or fights, leaving the spell to chase down a fleeing enemy with the high speed boost. Vendetta is also a great escape mechanism if need be.

• Spiked Carapace is arguably one of the best defensive skills in the game that when used properly can disrupt whole teams and save you from many deaths despite your low health pool. Sometimes it might be a good idea to max it before your Mana Burn as the increased stun duration will come in handy when combined with your Impale.

• Due to its scaling Mana Burn becomes more powerful as the game progresses as it reduces the mana and damages its target based on their intelligence. High intelligence gaining heroes with low health pools such as Pugna, Enigma, Silencer make excellent targets later in the game. 

source: http://dota2.gamepedia.com/Nyx_Assassin