Malthael was, perhaps, Rathma's second worst nightmare coming true (the first being...well actually it was not the second, or even the third. He had too many nightmares.) The angel sucked upon the power of Death without the consideration for what it meant. He had no idea what he was doing with himself, or the souls (there was so much one could do with that much soul-energy - and he was merely here to destroy them all-). His behavior was erratic and crude. Did he know that he was suckling upon the very veins of life? Likely not. Rathma was rather certain the angel only saw the souls as a means to an end. A power source.
Wisdom indeed.
Unfortunately, fighting the angel left little room for insulting his practices and how rudimentary they were. When he had breath, it was used for words of power. Around him, the (smaller-weaker-fragile-strong) Neo-Nephalem darted about. They'd come a long way, but had farther to go for something like the Angel of Death. All together, they had a chance - all together, they'd defeated Diablo after all. But they were not all together here.
Malthael was quick, he was strong, but Rathma knew his kind well. Death was his wheelhouse, the Angel was merely a guest.
A foolish guest too. Malthael had no idea that there were other Neo-Nephalem about, so focused on the opponents in front of him. Rathma wondered if he'd always been this narrow-minded, or if there was something else at play here. It seemed unlikely that this was what had out-thought and out-maneuvered the likes of The Three for so long. It seemed...very much like what had become of his father. Were angels simply fated for insanity? Perhaps they couldn't handle any kind of power.
Perhaps he'd bring it up with Tyrael, if they lived. Right now was really not the time for musings.
They tore into one another. Malthael was bigger, Rathma was nimbler. They drew on the same power-source, and the souls of the Dead were not willingly obeying the angel. Around them, arcane and arrows lit up the air in violent colors. There were shouts and curses and spell-words thrown about, as everyone fought for their lives. One slip-up meant Death - permanent Death, and their soul would go to fuel the monstrous angel that they opposed.
How long could they do this? The dance of blades and power and Death?
Forever, presumably, or until someone simply dropped.
They all recognized this flaw in the battle, all knew the match was even. If one of them fell, they all went down. They needed only take down one angel, but Malthael had decided not to take chances...
Except.
Except he'd already given the Neo-Nephalem all the chance they needed to steal the Black-soulstone out from under him. As one group had fought him and kept his attention, the rest had torn through the fortress until they'd reached the Black Soulstone, resting in the chamber where the Worldstone had one lain.
Rathma cackled in the angel's face, through broken and bleeding teeth, and was nearly cleaved in two for it. (Since when were shotels cleavers? He'd have to study those blades later…)
Without the stone to contain the souls, it was simple work for Rathma's children - the Priests, the Chosen, call them what you like - to dismiss the multitude of ruined lives. As they escaped into eternity, they took Malthael's power with them. His scream of rage was beautiful (if shrieky and awful) to hear.
Malthael was alone then. No souls to draw on, no Soulstone to consume, and a host of angry mortals arrayed against him. He did not willingly surrender, and Rathma was not the slightest bit surprised. He was still a vicious opponent, and now he was angry. The fury he flung at his adversaries should have killed them all, but they miraculously kept surviving. Kept coming to one-another's aide. Kept fighting back.
They were winning now. Slowly. They were bleeding. They were battered. There was no retreat though, no option but to keep on fighting, fight until Malthael was dead or surrendered.
In the end it was not Rathma nor the Neo-Nephalem that felled him.
That honor went to Imperius.