Alduin gradually rights itself, beating away the blows of the warriors and prepares to take to the sky.  Tirisi lets his shield fall to the floor and grabs the singular talon of the wing he had been attacking moments prior; as the dragon beats its wings, he is taken up with it, flailing helplessly along with the motion of the limb.  As Alduin begins to dive toward the ground, and so flattens its wings, Tirisi swings himself onto the fleshy membrane.  Gripping the talon with his left hand, he brings his sword downward to pierce the wing, but it snaps sideways out of his hand as it meets the seemingly impenetrable skin.  Tirisi scrambles to catch the blade, but it’s gone in an instant, fallen into the mist below.  Alduin, now keenly aware that it has a being atop it, begins to climb, once again sending Tirisi into fits of barely restrained freefall.  Soon frustrated, it brings its head backwards and attempts to bite Tirisi, giving him freedom to throw himself from the wing of the dragon to its head.  There’s a screech and violent thrashing as Alduin tries to throw Tirisi from its head.  It turns itself on its side and scrapes its back against a cliff face in desperation, but achieves nothing.
Now sitting astride the beast’s neck, comfortably between its horns, Tirisi reaches down to his belt and retrieves the knife he’d bought in Solitude, he searches briefly for the gaps between the scales and jams the triangular blade in as he finds them.  Each stab elicits a deafening scream from the dragon, which now has given up on the figures below entirely.  Tirisi digs the knife into the skin a final time, and attempts to pry the scale off the beast.  The scale refuses to move, it merely causes Alduin more and more pain.  The dragon brings its head back as far as it can and torches its own wing as it attempts to free itself, before resorting to climbing as high as it can and plunging downward; the sudden change in direction, combined with the wind blows Tirisi flat against the neck of the beast, though he remains atop it.  As he opens his eyes, he sees a huge body of water screaming toward him.  He closes them again, bracing himself.
They hit the water.
The force of the impact visibly cracks the horn to the left of Tirisi, who’s bent himself forward and is desperately gripping the scales around his knife.  As he opens his eyes, he can see vast plumes of steam flowing to either side of the dragon’s body as the flame in its mouth evaporates the water around it.  A few forlorn bubbles seep through the mouth-gap of Tirisi’s mask, as the dragon continues to plough through the water.  After a few minutes of swimming in circles, Alduin turns its head upwards and begins swimming up to the surface.  As soon as it breaches the surface it begins to fly back toward land, seemingly satisfied that Tirisi had drowned.
Alduin returns to above the Hall of Valour, and once again begins to make attacks toward the warriors below; as it brings itself around for the third pass, its right eye is split by the blade of Tirisi’s knife.  It immediately goes into a spasm, throwing itself wildly off course, spewing blood and chunks of eyeball backwards onto Tirisi, who brings the knife back for a second strike, this time at the other eye.  The dragon is hit once again by the shouts of the nords, and falls from the sky, crashing near to where it had initially; this time, however, the crash has visibly snapped one of its wings.  Tirisi rolls from the head of the dragon and attacks the underside of the neck, repeatedly stabbing, but failing to penetrate.  The dragon’s head turns, and Tirisi sights the opening in the armour, no larger than a Septim.  As he brings his arm back to strike the weakness, he hears the scream of a man, rather than a beast, getting quickly closer.  He turns to look and sees Benor sprinting at Alduin, axe held high, poised to strike.  The instant he’s close enough, he slams his axe into the shoulder-scales of the beast.
There’s a moment of total silence.
Tirisi bolts from beneath the dragon, grabbing his shield from its resting place a dozen feet from where the beast now lies.  Alduin brings its head around and throws Benor’s axe across the valley, and brings its jaws onto his now naked shield arm.  It bites.  The teeth easily part the armour, flesh and bone, and tear the arm from its socket.  Benor goes pale, and falls backward, blood spurting regularly from his wound.
Alduin brings his head back.
Tirisi sprints back toward the pair and throws himself to the ground, shield roughly pointed toward the dragon’s maw.
The maw opens.  A gout of fire bursts forth.
As the fire clears, Tirisi still holds the shield, now glowing fierce red rather than its usual mellow green.  With his arm badly singed, Tirisi drops the shield and charges back toward Alduin’s underbelly, knife at the ready.  Before the dragon has the chance to react, the blade has pierced the hide.  Tirisi throws the knife to the ground and rams his hands into the hole, tearing it open.  He reaches in, blindly grabs the first thing his palm meets and rips it from its place.
Alduin now unleashes a scream unlike any heard before.  The world around the battle shakes; all who hear it have their ears go numb and deafen; the still writhing corpse of the dragon turns blinding white and vanishes, leaving nothing more than the bloodied and beating heart that sits in Tirisi’s palm.  He mirrors Alduin’s final scream, holding the heart up to the sky, before bringing it down, staring at it, and sinking his fangs into it.  Once the heart has been drained in its entirety, Tirisi begins to bite through it, swallowing as he does so.  Before long the heart is gone and the only remnants of Alduin are the blood on his hands, face and blade.