I actually thought up a champion with a skillset that pretty much matched your criteria the day before you posted this. So here it is, my interpretation of Qurego'mas's skillset.
Passive: Void Corrosion: When a unit is hit by one of Qurego'mas spells it becomes corroded increasing damage done to the target by 1% (1 stack) 3% (2 stacks) 5% (3 stacks). Lasts 8 seconds, stacks up to 3 times.
Void Missile: (Range: 625 Cost: 80 mana) Qurego'mas unleashes a missile empowered with void energy dealing 70/120/170/220/260 (+0.7 per ability power) and lowering their damage dealt by 10/12/14/16/18/20% for 3 seconds. 11/10/9/8/7 sec CD
Presence of Mind: Passive: Qurego'mas's mental presence generates a bubble of force around him, shielding him from damage, blocking 40/80/120/160/200 (+0.8 per ability power). The shield is refreshed if Qurego'mas has not been hit for 10 seconds. The shield cannot regenerate when this ability is on cooldown.
Active: (Range 2000/2100/2200/2300/2400/2500 Mana Cost: 100/105/110/115/120) Qurego'mas detonates his shield to send a pulse of mental energy to the area around him revealing enemy champions for 5 seconds. Enemies are made aware of his presence as well. 50/45/40/35/30 second CD
Mind Blast: (Range:1000 Radius: 350 Mana Cost: 80/90/100/110/120) Qurego'mas concentrates his mental energy at a target location. After a short delay (.75 sec) the energy is released. Targets hit by the blast are blinded for 2 seconds and are dealt 90/140/180/220/260 (+1.0 per ability power) 10 second CD
Void Glimpse: (Range:675 Mana Cost: 100/150/200) Qurego'mas takes advantage of his opponents void corroded body, opening a small window to the void and materializing their darkest fears. Deals 100/150/200 (+0.4 per ability power) 150/130/110 second CD
1 Stack of corroded: Deals 2x damage and slows targets attack and movement speed by 25%
2 Stacks of corroded: Deals 2.5x damage and slows attack and move speed by 30% in a 500 radius around the target
3 Stacks of corroded: Deals 3x damage and slows attack and move speed by 35% and fears targets for 1.5 seconds in a 500 radius around the target\
I'll add notes about it later have to go now
Ratios need work i know