Hi! I am Markomeister, 19 year old mid lane main. I am Zed and Vel'Koz main currently in Platinum Elo. Currently ranked #166 Vel'Koz EUNE (Top 1.3%) and #1425 Vel'Koz overall (Top 1.1%)

Why Night Harvester ? With Night Harvester we get extra poke damage early on (which scales) we desperately need because we need to combo our abilities do to damage. With Night Harvester not only our Q poke is stronger, but it also makes our W pokes notable for our enemy.
That is way this build is aggressively focused build made to harass our enemy laner and be used as main source of damage in our team.
Since we drop Luden's Tempest we will have early mana problems we are going to fix with Tear of the Goddess into Seraph's Embrace

Our first rush item is Night Harvester. Ideal way to start the game is to recall when we can instantly but Hextech Alternator and hopefully teleport back to lane for farm or teleport top/bot to grab some takedowns.
Second back should be our mana issues helper Tear of the Goddess that we are going to upgrade into Seraph's Embrace
After we get our 2 main items, all other options are situational ( Rabadon's Deathcap excluded, you will want to have it mid to late game). You will either go for Morellonomicon if you need anti-heal or more balanced option like Cosmic Drive.
Zhonya's Hourglass and Banshee's Veil are our defensive options that hopefully you will not need to get and only focus on pure damage and destruction

Default option for summoner spells are Flash and Teleport. Teleport is strongest summoner spell in game, both for defense and offense. You can use it to get back on lane fast, engage other lanes, protect objectives or even save your life if you use it properly in a fight
Other option is to replace Teleport with Barrier. You should use Barrier only if matched vs high engage enemies like Zed, Talon or Katarina that can 1shot us

Since we have very good gap closing, area controlling abilities, most of the champions cannot hurt us if we don't allow them to. That is why our main problem are champions that can close distance between us and them very fast and engage, like Zed, Talon, Katarina or Kha'Zix, in which situations we can only try to hit our E and run away or Flash since we have no movement (That is why Cosmic Drive is very good for us since it gives us movement speed) and combined with Night Harvester movement speed buff we get if we land our ability, we will have enough time to escape.