This is a guide for Vel'Koz. Here I will show you one way of how tentacles can destroy whole teams. Mind that there are more ways to play this champion. This is the aggro sustained damage way.

Pros:

• High sustain damage.
• High survivability.
• Low CDs.
• Low ability mana costs.
• LAZORS!!!

Cons:

• No mobility.
• Susceptible to bursty assassins
• Can be hard to get used to.

This are the runes:

Armor seals and magic resist glyphs are my favorite on any champion. We will make this runes even more cost efficient straight off the bat. I'll show you why/how in the item section.

Some of you may strange the ad marks, but fear not. Ad mark are very powerful. They will help you farm, poke and continuously deal damage while you wait for your cool-downs.

This is the mastery tree.

Well now, here is where the magic happens. The masteries and runes join up with your items to bring the power of the void upon runeterra.

Start up items:
In the most recent patch, the ruby crystal became cheaper. You can now start with it right off the bat and rush to Vel'Koz's core item: Rod of Ages. It synnergises very well with the stats we bring from the pre-game. It gives us sustain, mana and survivability.
First back:
If you have this after your first back you are doing good. If you have the catalyst only you're not doing bad. Otherwise, step up your game, and never forget about the Stealth Ward.
Core Items:
This will be your core items. Damage and survivability combined. Your continuous siege with your spells will wither the enemy team down and you won't have to worry much. Usually you won't have mana problems, but if you do, ask your jungler for the blue buff.

From now on you're on your own. I personally like building Will of the Ancients and Rylai's Crystal Scepter so I can keep aggroing and sustaining myself (spell vamp and mana regen from WotA) and having a lot of kiting potential and easier ability landing (slow from rylai).

You don't really need to build a straight defensive item because you will have tons of health, however, if you get focused a lot, pick a defensive items on your sixth slot in conformity with your needs.

[[tectonic disruption] is maxed last. You use it to do the WE combo and to escape. It's a utility spell, not a damage one.

Plasma Fission and Void Rift go hand in hand.
You will use plasma fission to poke and kite, while W is for farming. However, this is not set in stone. The best Vel'koz will be the one who is better at using his W.

Flash and Barrier

You already have Summoner's Insight to help you have the cool-downs refreshed more frequently. You'll need them.

Ignite is not very good because you will usually want to be outside of it's applicable range.

Get Jarvan IV on your team. Both of you press R. Watch bi**es melt.

You'll want to have initiators on your team. AOE stuns such as Amumu and Sona ultimates are devastating when comboed.

Champions with slows are also very good with you. because the slows will keep them at bay for your barrage and also keep you safe.

Plasma Fission is a very unique ability that has a lot of potential. You can hit up to three enemies with it and the slows is a big help. Use the geometry of the spell and detonate it at the right time.

Life Form Disintegration Ray is a VERY powerful ability. Its base damage is huge and it penetrates everything. If you hit all of it on several champions, the team fight will turn into your favor.

Have fun collecting knowledge!