Alright, here we go. My summoner name is Afterman, it's changed a lot actually, and some of you may have played with me, if you play on the NA servers. This is my first guide, i feel like i've probably put a lot more in the notes for my purchase orders than i should've, im going to get into a little bit more detail regarding why i chose the items in the items section of this guide. But before i got into that, i thought i'd let you guys know who i am. I'm not a pro player or anything, but i have something around two thousand games as a mid laner under my belt so i know my stuff when it comes to AP casters, and mid lane mechanics in general. Enough about me, this is about Vel'koz. Vel'koz is an AP caster champion, typically played as a mid laner. Some of his distinguishing traits are that every one of his abilities are skill shots, so if you're used to someone like ryze, katarina, annie, or any other point-and-click champion, his kit will most likely be a bit more difficult for you to master. He makes up for this higher skill cap with extremely high damage output and a passive which deals true damage based on if you land your abilities successfully. Another notable trait about Vel'Koz is his range. the base range on his q, w, and e are comparable with the ranges on champions like Syndra, or Orianna. This makes him very good at poking from long distances, and, if you're good at timing the split on his q, you can actually increase its range by 33% and maneuver it around minions. The third most notable trait is his running animation. really, get homeguard boots, you'll see what i mean.

The runes that i play on him are fairly standard, i typically like to play assassins as mid laners (leblanc, katarina, diana, just to name a few) so normally, i run armor seals as opposed to Mana-regen seals, but on casters such as vel, malz, lulu, veigar, etc, i do run scaling mana regen seals. magic pen runes are amazing in general, the AP quints give some good starting damage, and the magic resist glyphs help with trades in the mid lane (in most cases, sometimes you come across an ad assassin mid laner, in those matchups i switch out for my Assassin page and opt for the extra tankiness over the mana regen)

I dont know how to make the guide fancy with photos and stuff, so im just going to refer to the notes i left in the purchase order.

Tear of the Goddess is the one item you want to get as soon as you possibly can. a seraphs embrace at 25 minutes is extremely strong. in addition the thousand mana, paired with a ROA this gives vel the ability to just fire off his abilities with little to no regard for when he will run out of mana. in addition, the active on seraphs embrace can be an enormous life saver.

ROA (Rod of Ages) gives good early sustain through Catalyst, then when the ROA itself is finished, it will start giving Vel some tankiness. this will allow him to poke enemies down in prolonged team fights without instantly being nuked down by an enemy. Vel's passive, organic deconstruction, benefits from prolonged fights, because he gets to put more stacks of his passive on his enemies, slowly chunking them down with true damage.

Optional item choices: Zhonya, Morellonomicon, Haunting guise/liandry's torment
-- Zhonya is a good item, thought not super beneficial on Vel due to his lack of escapes. the armor helps against hard AD teams which is a good redeeming factor for the item.
--Morellonomicon is a much more Gold-efficient item (probably the most gold efficient AP item in the game) that helps counter act the healing effects that those AD champs would get from the lifesteal they build (talon with his hydra/bloodthirster, zed with his BOTRK etc) this item is very good on Vel due to its 20% CDR and mana regen as well (my personal choice if you had to pick one).
--Haunting guise is a cheap item that one could build if they were falling behind or if the enemy started stacking Magic Resist early. upgrading it Liandry's is almost not worth it due to the item being quite expensive at 2900 gold for only 50 ability power and 300 health (ROA gives 60 AP, 450 health, 350 mana, and gives more and more AP health and mana over time and its actually cheaper than a liandrys) the only instance where i would justify purchasing this item is if their whole team began building warmogs, since it does damage based on max health.

Void staff/Abyssal Scepter: Whichever one you get depends on the enemy team and how far ahead you are. if you're able to gun down 3 of their team with just an ulti, i'd go ahead and purchase a void staff to see if you could do some tank-busting. if, however, their mid laner is ahead on you, i'd purchase an abyssal scepter.

Rabadons: Gives you a massive increase in Ability power 120 base plus another 30% of what you already have. pair this with the seraph which gives you ability power based off of mana, and your stats get really high really fast. if, however, you're in a losing position, you can purchase a zhonya in this items stead due to it giving a good amount of armor and an active that prevents targetting or any form of damage.

Ending item:
-- Zhonya, if you're way ahead and you can purchase a zhonya without a worry in the world, do it, and enjoy melting everyone.
-- Guardian Angel, this is a bit of a go to for most champions end game item. this is due to the item giving both a good amount of MR, but also armor, and extra survivablitiy, which can win team fights. personally, i dont like this on Vel, because he has no actual escapes, making this just a delay if the enemy team is stomping you. i would only get this if their AD champions were ahead or if my lane opponent was an AD mid and you really needed the armor.
-- banshee's veil, my personal favorite on Vel, simply because it gives health which gives him the ability to poke for a longer duration, and the longer he continues to poke, the higher your chances of winning are. the reason why i like banshee's veil so much, however, is because of the spell shield. as soon as vel roots himself to start channeling his ulti, he becomes target number one for any form of cc to cancel the channeling. the shield on banshee's veil can stop one.

Alright, as far as the skill sequence goes on Vel'koz, it really does depend on your lane opponent. against someone like ziggs, i would opt to max q over maxing w, but against most other lane opponent who have shorter ranges than vel (malzahar, leblanc, kat, zed, gragas, kass, etc) i'd max my w because it will allow for higher damage output because if you land it consecutively you get your passive stacks, and in the low levels true damage of any amount hits very hard. in addition, Vel's W passes through whole minion waves, allowing you to punish melee champs for last hitting, and even on other ranged champs, you'll be able to apply some stacks on them and bully them out of lane. The primary combo i use with Vel is a w-e combo (really its more like e and then immediately cast w on them since it has no cast time) this combo can bring a full health champ to around 30% since it guarantees a full Organic Decontruction rotation landing if you land the knock up. i use q as a harassment tool and for its slow primarily so i dont really level it up until i need to. of course max Life Form Disintegration Ray as it becomes available. it can win team fights by itself. an example would be if they group up, just get far back and fire it while your team fights. it forces them to disengage, focus you down, or to spread apart to mitigate the damage. typically, in my experience anyway, they tend to spread apart. when they do that allows your team to single out targets easier. more on that later.

See Skill Sequence, i think i covered most of it there.

Okay, i dont know how to do the whole fancy pro/con list like most guides
Pros:
Longer range than most other champions
High damage output throughout the whole match due to his passive
his q and his w's cast time are short and non-existant respectively, making him great at kiting.

Cons:
not mobile
you have to be able to land his skill shots to do any damage
higher skill cap than some champs
he's not as creepy as his splash art suggests.

There are a few champions that vel'koz works very well with.
Jarvan IV- cataclysm makes ulting a whole team very easy, in addition you can knock up their whole team while they're stuck in it with him.
Sona- Mass stun due to cresendo. depending on their position, a good Vel and Sona could win a team fight by themselves
Amumu- same sort of reason as the other two, it removes the enemy teams ability to escape from the ultimate.

Alright, thats about it for my first guide. i hope what you picked up from this was that vel is a skill-shot only, long ranged caster, who has very high damage output if you can combo his abilities right. in addition, i gave some examples as to who works well with him, which should help if you're duo queuing or in a team.
Now, go, and explore.
You'll learn so many new things, but your enemies will only learn the meaning of pain.