Vel'Koz

Middle

Deadly Disintegration Damage

Long range lane harassment poke.

Sorcery

Empowered abilities and resource manipulation

Arcane Comet

Hitting a champion with spells hurls a comet that deals adaptive damage at them. Landing more spells reduces Arcane Comet's cooldown.
The Ultimate Hat

Gain cooldown reduction and max cooldown reduction on your ultimate. This is increased further each time you use your ultimate, up to a maximum.
Transcendence

Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Scorch

Your first ability hit every 20s burns champions.

Inspiration

+20 Ability Power or +12 Attack Damage, Adaptive

Magical Footwear

You get free boots at 10 min. but you cannot buy boots before then. These boots give additional MS. For each takedown you acquire the boots 30s sooner.
Cosmic Insight

+5% CDR
+5% Max CDR
+5% Summoner Spell CDR
+5% Item CDR

WhyArcane Comet

Arcane Comet is a great keystone choice for Vel'koz as it reliably empowers his ability to poke in lane. Thanks to the slow from [Q] and [R], and the small knockup from [E], you should be able to reliably land Comet if you are hitting your abilities.

WhyThe Ultimate Hat

With your [R] up on Vel'Koz you can constantly threaten the potential of a 100-0 one-shot if you're able to land a full combo. More [R] uptime means more outright damage threat.

WhyTranscendence

Transcendence allows you to hit 35% CDR with just Morello's and Cosmic Insight, allows you to spam your abilities for poke damage and kill pressure.

WhyScorch

Scorch further empowers your ability to poke and harass in the laning phase. If you're able to proc Scorch on cooldown several times before hitting level 6, you should be able to keep your lane opponent in kill range when you skill [R].

WhyMagical Footwear

Magical Footwear is a great rune choice on just about any champion largely for its raw gold value with no input. Think of it as effectively gaining a kill's worth of gold at 10min.

WhyCosmic Insight

Cosmic Insight is an all-around great rune choice on any champion, especially poke champions who are constantly looking to deal damage from range when their abilities come off cooldown.