While the flasks did not directly hit their mark, the splash from them certainly did. The first estus passed through the armor, shattering on contact with the bones underneath, while the second barely missed, hitting the outer shell. The damage from the combined flasks was potent, decimating the remaining innards of the beast, and burning away at the pulsing core that was his heart. Through the pain of death, Diabolus swung his massive hammer, carrying the momentum down to meet the impact point. With the deafening crack of a shock wave, the maul clashed with the ground, causing a violent upheaval of rock and showering the area with the buildings' debris. The eruption quaked the city, and the ground collapsed around the crater the beast stood in as subterranean tunnels and caverns crumbled beneath the pressure. Like an explosion, the wave carried into the surrounding blocks, covering four hundred feet away in all directions. More buildings collapsed under the shock and the remaining roads split into ruin.
As the dust settled, the form of Diabolus could be seen gripping the hammer tightly, hunched over with barely anything holding him together. The now faint red glow of his heart ceased to burn, and the corpse of the beast crumbled apart, turning into a thin black mist before vanishing completely.
The Maul was defeated, but far from gone. It would return, one day. Hundred, perhaps thousands of years from now, in another world, or another Time. Such is the way of The Eight.