I have a few ideas I felt like I'd toss out there for you guys, so I'll just start out with my thoughts on a "core" kit mechanic to build around. It's difficult for me to really fill out the rest of the kit without some real thematics set in stone about the champ, so I'll discuss that a bit more after the kit ideas.
Innate - Rift Riot
Damaging a champion grants (Prisoner) a stacking 5% attack speed boost for 10 seconds. There is no limit to the amount of stacks that can be gained in this way.
Q - Hundred Blades
~15s base cooldown, scaling downward with the amount of attack speed (Prisoner) currently has from all sources
(Prisoner) targets an enemy within melee range and strikes four times, once with each weapon. Each strike deals 20% of (Prisoner)'s total attack damage plus a small amount per point in the ability, and can critically strike. On-hit effects other than critical strikes can only occur once from Hundred Blades, but each strike builds a stack of Rift Riot.
R - Rebellion
Vowing to see such worthy opponents slain at his feet, (Prisoner) temporarily removes the shackles that bind his arms, unlocking his true potential for a time. During this time, (Prisoner) ignores the attack speed cap and gains 5/7/10 stacks of Riot. Additionally, each strike from Hundred Blades during this time will be a critical hit.
As far as lore and thematic stuff goes, my first thoughts are that I don't want this guy to be a stereotypical musclehead type of character. The four-armed thing pretty much says "I'm an idiot" in most cases, I'd like to push for a bit more of an educated approach here. Like, starting with the basics.. instead of him just smashing through a wall to get out of his prison, I feel it would be much more interesting if he was the type to be able to design and stage an escape rather than relying on strength for everything. I feel like that would fit more with the whole "Fastest attacker on runeterra" thing as well, as I haven't known many muscleheads to be extraordinarily fast.
The other two big questions I'd like to brainstorm a bit about are "What crime did he commit?" and "What is his motivation to still be wearing the shackles?". Does he know that he's guilty and still bear the chains as a way of atonement? Or is the extra weight just to make his fights more interesting, since fighting without the weights lets him dance circles around everyone else?