overload is used as a massive damage filler spell to burst the enemy down and reduce the cooldown of your other spells.
rune prison is a significant damage snare
spell flux is a good aoe spell and an amazing single target spell if utilized well.
desperate power gives you aoe damage which is good in teamfights and spell vamp which gives you a bit more survivability

its not too hard to figure out why I chose the items. their pretty self explanatory

build one:
build one is just a fun build to maximize mana fast
build two:
this one I actually use a lot and it is very fun. it focuses everything on spell flux. since spell flux comps better with ap than mana I decided to focus more on ap this time.
Magic Damage: 50 / 70 / 90 / 110 / 130 (+ 35% AP) (+ 1% of max mana)
Maximum Magic Damage to One Target: 150 / 210 / 270 / 330 / 390 (+ 105% AP) (+3% of max mana)
Maximum Magic Damage to One Target with Desperate Power: 225 / 315 / 405 / 495 / 585 (+ 157.5% AP) (+ 4.5% of max mana)

Resist Reduction: 12 / 15 / 18 / 21 / 24
so this is amazing in a 1v1 fight with your ult.

build three:
this is my favorite and most commonly used build, u can use it if u like... this is just my preference.