Marsh - also known as Ironeyes, due to his hard, piercing gaze - is a half-skaa Seeker; he is a Misting that can only burn copper, which allows him to detect allomancy being used. Once the leader of the skaa rebellion, he resigned when he felt like it was going nowhere.
His brother, Kelsier, contacted him and pulled him out of retirement to help overthrow the Final Empire he had been fighting against for so long. Marsh joined the Obligators as a mole, infiltrating the combined theocratic and bureaucratic centers of the Final Empire. During his time there, he attracted the notice of the Canton of Inquisition, who decided to recruit him.
This entailed Marsh being turned into a Steel Inquisitor, a process involving the deaths of multiple people and completely bathed the room in blood; this gave a different twist to his nickname, since he now had steel* spikes driven through his eyes. After the Lord Ruler was killed, he wandered around the former Final Empire to try and piece together the story behind the man's ascension. This led to the freeing of Ruin, a Shard of Adonalsium. Ruin then took over the minds of anyone affected by Hemalurgy, either directly or through hallucination-based manipulation, including Marsh himself. Marsh later became Ruin's right-hand Inquisitor, gaining more than twenty hemalugic spikes, boosting his Allomantic and Feruchemic strengths to extreme levels. After Ruin's death and the ascension of one of his old friends as Harmony, Marsh regained his mind and traveled the world serving Harmony of his own will.
*an alloy of Iron, for those who don't know basic metallurgy
Dalinar and Marsh both enter the Arena, exiting from opposite doors. Dalinar draws his warhammer, and looks at his opponent. He quickly takes another couple steps back, somewhat terrified of the spikes visage in front of him.
Marsh smiles and draws his axe, and rushes forward to fight. Dalinar recovers and gives a solid blow to Marsh's chest - on a normal man, it would be a kill, but Marsh is compounding gold. Marsh takes the opportunity to try a strike to Dalinar's chest, but it gets deflected by the Shardplate. The warriors disengage, and circle each other for a couple seconds.
Dalinar starts to feel the Thrill, the hereditarty bloodlust in his family.
He leads off the attack in the second clash, charging forward with his warhammer. He puts all his strength behind it, aiming to break the obsidian blade of Marsh's axe. Marsh burns iron and Pulls the warhammer, causing the head to slam into his chest rather than the axe. Still compounding gold, he doesn't suffer a scratch. He lashes out with his axe at Dalinar's unprotected head. Dalinar moves out of the way in time, and the axe leaves a gash in his helmet. He breaks off and tries again to break the axe, and succeeds in snapping the comparative4ly brittle obsidian head. Marsh, having noticed the glowing spheres in Dalinar's armor, tries to use the wood handle as a battering ram to break it.
Dalinar responds by grabbing the handle and crushing it in his gauntlet.
Marsh quickly Pulls the Warhammer out of Dalinar's grip, compounding iron to give his Pull some extra weight. As the Warhammer flies out of a startled Dalinar's hands, Marsh rushes in with his fists. Dalinar barely has the time to make the summoning gesture for his Shardblade and block with his free hand. He knows that he will win after ten heartbeats, when Oathbringer will materialize.
Dalinar has taken several hits already on his Gauntlet, and some have gotten through on his Chestplate.
Marsh starts compounding pewter, steel, and bendalloy, giving him increased speed, strength, and energy.
Dalinar has managed to get a punch or two in, but they barely effected Marsh. Several punches have gone through, and his Shardplate is cracking on his arm. The adrenaline pounds through his veins harder then ever.
Four. Five. Six.
The plate on his arm appears to be turning liquid.
The Shardplate on his arm breaks.
Marsh draws a spike form his robe, and starts stabbing towards Dalinar's now unprotected arm.
Oathbringer shines in Dalinar's hand, and he cuts the spike in half. He starts pushing Marsh back, keeping the blade moving constantly. Marsh, burning Atium, is just fast enough to evade Dalinar's strikes. In a sudden burst of inspiration, he stops Atium, and burns Duralumin and Zinc.
Dalinar is stopped in his tracks be the mental blow. Not only was the Thrill gone, no longer egging him on to fight, but all his emotions were just... gone. Marsh uses the opportunity to land a powerful roundhouse kick, and Dalinar is sent flying, and drops his Shardblade. He gets up, recovers, and starts summoning his Shardblade again while Marsh charges him.
Marshi is getting closer. Dalinar aims a kick at him.
The kick connects, stalling Marsh.
Marsh sees the fallen warhammer and smiles.
Marsh Pulls himself towards the warhammer.
Five. Six. Seven.
Marsh reaches the warhammer, and Dalinar starts to run towards him.
Marsh karate chops the warhammer's head off, while Dalinar charges at him.
Marsh holds the warhammer's head at chest level.
Oathbringer springs into Dalinar's hands.
Marsh Pushes the the Warhammer's head towards Dalinar, burning steel and compounding iron. Dalinar blocks with Oathbringer, but the chunks of the head keep flying at him. Thanks to his enhanced weight, the two pieces of weighted metal push Dalinar into the wall, impacting with enough force to break the Shardplate on Dalinar's chest and impaling him , holing him to the wall. Marsh walks up, and takes out a spare spike - one without a Hemalurgic charge. He stabs Dalinar in the heart with it, and removes it. After examining it confusedly, he walks away, leaving a bloody, broken body behind him.
Marsh won almost entirely due to his Hemalurgy. Both warriors had one weapon that was overwhelmingly powerful; Oathbringer for Dalinar and Hemalurgy for Marsh. Hemalurgy just gave him more combat advantages than Oathbringer did for Dalinar. Save those, they are very evenly matched.