Welcome to my build.
DOMINION is one of my favorite matches and I play none other than the Rift Ninja himself Kassiden.

Unlike Summoners Rift Kassidens weakness for health and mana has been made Null in Void with the starting cash of dominion and directly making him level 3 already turning him into a Force not to Underestimate.

(NOTE: I have fortify for his summoner abilities wait till they fix it...)

Not rocket science here...

The reason why I chose what I chose is because thats the BOTTOM line cause StoneCold sed SO!?

but really its logical is it not... Dominion reflects more on a champions strengths, its trying to balance Offensive with Defensive.

Kassiden Early game Runes makes him deadly to get close to, YES he will be slow (more into using that to your advantage later.)

MR(Magic resist) per level Seals cause that's all I had, and I don't care LMFAO! its just logical with the Rune set up intergrating it with the items makes him...BALANCED!?

Of course another NO BRAINER...

Basic AP champ Mastery Set up...

CtP ALWAYS first choice WHY!? more Health, Mana and its Passive BALANCE
change to
RoA: A very Balanced HP,MP,AP build up over time a must get for Kassiden
Grab
Sorcerers Shoes: Extra Damage=Now a very bad threat (Target)
Next
Glacial Shroud: CD,CD,CD!!! YAY more armor and Mana= Rift Walk upgrades to Rift Stomp (Target)
change to
Frozen Heart= Don't bother hitting Kassiden retreat! WHY??? Rift walks Passive go and read then Perfect the Rift Walk know the way of the Void Gaurdian.

REMEMBER TACTICS AND CLEANSE WILL DEFEAT THE TEAM PLAY SMART!!!KNOW WHEN TO ACTIVATE CLEANSE AND SOFTEN UP THE ENEMY TEAM BEFORE YOUR TEAM GETS THERE!!!

This is entirely up to you... could be anything you want sometimes i need to protect myself from carries so we all know what to buy AYE if not check a forum on it please. Im just showing my Balanced build and thats pretty muj it my runes, masteries and Items...

I focus on Mana, Armor and CD to get the most out of Kassiden. Rift Stomp if timed correctly could devestate a whole enemy TEAM and send em running with you and your carries catching up and mopping it up.

1v1 are also his specialty I love the skill shot champs cause they are easily predictable and you could literally juke the shot with Rift Walk(100 Magic Damage), force pulse(Slow), Null sphere(Silence), Auto hit, Riftwalk(200 Magic Damage) Behind cover wait for CD on Null sphere Rift Walk(300 Magic Damage) to opponent, Null Sphere(Silence), RIFT STOMP(400 Magic Damage)Yummy!!!

This Build Balances him he Has to be in the middle of the Map so he isn't to far from anyone. Once Core build is achieved Kassidan can Leap into action if an enemy is capturing a turret Just keep Rift Walking towards target each Rift Walk gets stronger but absorbs more Mana aswell as soon as your next to target Rift Walk will Qualify as a Rift Stomp and Hurt the shizznit out of the enemy to cut his losses and Run and that's not even adding the damage from Force pulse and Null Sphere,