Sentinel, the Invincible

One Sentence Summary: Ridiculous Duelist Tank, limited by Mobility

P: Best Offense:

Sentinel deals 30% of his bonus armor as bonus physical damage on his basic attacks. Sentinel also deals 30% of his bonus magic resist as bonus magic damage on his basic attacks.

1: Challenge:

Sentinel challenges a target within 20 range to fight him, taunting them for 1.5/1.75/2.0/2.25/2.5 seconds. Cooldown: 12 seconds

2: Counter:

Gain 3/6/9/12/15% damage reduction for 3 seconds. After 3 seconds, your next basic attack deals bonus damage equal to 70/85/100/115/130% of the raw damage taken, capped at 200/300/400/500/600 bonus damage (+20% of Sentinel's bonus health), lasting 3 seconds. Damage type depends on damage taken. Cooldown: 18 seconds

3: Tenacity:

Cleanse yourself of any CC and debuffs and gain 10/15/20/25/30% move speed for 3 seconds. Cooldown: 22/20/18/16/14 seconds seconds

4: Omniguard:

Gain 60/65/70/75/80% damage reduction and regenerate 15% of your maximum health over 3 seconds. Cooldown: 30 seconds

Pros: Ridiculous tankiness, powerful dueling, nigh-invincible tankiness, lots of itemization options, absurd tankiness.
Cons: Slow early jungle clear, pretty slow.

Playstyle: Get into the jungle, get some armor items, and stroll into the enemy jungle to beat him to death or drive him away from his camps. Keep building tanky all game as you ocassionally gank weakly, eventually transitioning to teamfights, where you are pretty much a constant source of damage who diverts attention away from his team while remaining a strong damage threat.

Weaknesses: Being kited, despite the cleanse, since he needs melee range to deal damage. His abilities are very single target oriented, giving him a somewhat weaker late game than mid game.

Important question does he build physical or magical. If he builds magical he sucks but if he builds physical it could work. Problem with him being he has very little to no clear. One change you should make is for his counter ability give either the ending basic attack in a bakasura cone or all attacks for the duration impact a cone in a cleave type manner. If the bonus damage was treated the same way Xbals branching bola damage was in that it could crit then I could see this guy working. If not he would be kinda like Chaac 2.0 you couldn't kill him but he couldn't kill you.
With my ideas the way I would build him would be warrior tabi, Hide of the urchin, rune forged hammer, bulwark of hope, executioner, rage. Alternately you could swap runeforged for mystical mail. His best role would be solo as he couldn't keep up in mobility or clear with higher tier junglers.