Voldemort, since he lost very few Death Eaters and no important ones.
The enemy had only been stunned, since Dumbledore and his forces didn't kill, and the few Death Eaters who did die, did so by accident, rather than by design. Most were merely captured and shipped to Azkaban. Of course Voldemort promptly freed the Death Eaters that had been captured at the Ministry. They had remained incarcerated less than 72 hours.
Then Voldemort retaliated with "scorched earth" policy. In something that was obviously pre-planned, and pre-prepared, the entire building was destroyed by a gigantic magical detonation. Leaving nothing standing and no one alive. It had also taken out most of the people likely to challenge him.
Voldemort turned his eyes on other targets.
Diagon Alley. Hogsmeade. Both became victims of warfare. Nothing seemed sacred, and nothing remained untouched, and there was nothing that Dumbledore seemed to be able to do about the mounting death toll and rising rate of attacks. More, Dumbledore had refused to let Harry do anything but remain at his relatives, "where he was safe."
The International Confederation of Wizards had refused to lend aid as the situation had spun out of control. They had tried to blockade the entire island of Britain, hoping to contain Voldemort and his strengthening army of Death Eaters.
In response to the attempted blockage, the Dark Lord bided his time by focusing on Britain and waited almost three months, until after school had begun in the fall, and then he attacked three international schools in quick succession. Beauxbatons. Durmstrang. The Salem Academy. He offered those present the chance to join him, and then killed anyone who stood in his way or didn't join him at the schools. Then he razed the schools to the ground.
When the ICW convened in another emergency session to discuss how they were going to respond, Voldemort launched a surprise attack on the convention itself. In it he brought something that completely rocked the Wizarding World to its foundations.
Demons. Voldemort summoned demons.
No one had done that for centuries. Even the knowledge on how to summon them was thought to have been lost, and yet somehow Voldemort had found out how. The practice had been banned because of the massive danger involved in summoning demons. Not only to one's self and one's soul, but to the world as a whole.
No one could understand how he could have done such a thing. Because it seemed that he didn't summon just one demon, but summoned hundreds.
And because the knowledge was lost, no one knew how to effectively fight demons. Wizards were like lambs led to slaughter.
So quick and brutal was his attack that there had been only a handful of survivors out of over 300 wizards. Though Voldemort did suffer some Death Eater casualties in the battle. Dumbledore had attempted to duel Voldemort, only for Voldemort to match him, leaving the duel even, before Voldemort apparated away, the demons fading with him.
Voldemort next attacked Hogwarts. The last stronghold in all of Europe. Harry had watched as Voldemort had attacked the school with an army of demons at his back. However what was most demoralizing to everyone who had taken refuge there was when Voldemort arrogantly challenged Dumbledore and then outdueled and killed Dumbledore, before the demons swarmed the wards of the castle.
He and several friends escaped out of secret passages, but the school itself fell before the onslaught. And Harry and his friends were on the run from Voldemort.
The goblins had attempted to remain neutral in the conflict. After the slaughter at the ICW, and the attack at Hogwarts, Voldemort had marched to Gringotts bank and demanded they surrender themselves to him.
The goblins, being a proud race, and unwilling to surrender their freedom to anyone, had attacked Voldemort and his envoy of Death Eaters and demons viciously. Demons could be effected by magic but not killed, it was physical weapons imbued with magic that could kill them. And unlike wizards, they were adept at physical combat, their weapons attacking the demon forces effectively.
Voldemort escaped, but it had been the first setback that he had suffered, since several inner-circle Death Eaters he had brought with him, fell there.
He returned the next day with every Death Eater and a much larger contingent of demons in his force and assaulted Gringotts in a battle that lasted for over 24 hours. Voldemort crushed the goblins eventually. Determined to exterminate the entire race. However the losses his own group suffered were significant as well. Leaving him with somewhere around a third of his initial forces dead.
Harry missed part of that battle, having been chased through the countryside on a mission to find horcruxes. Not that he had anything more to work with other than vague hint, a snitch and a book of children's tales. All of which were given to him before Dumbledore rushed out to confront Voldemort, and die at his hands.
Harry couldn't help but clench his jaw when he thought about Dumbledore and his idiocy. His list of grievances with the old man was long indeed.
Voldemort had attempted to take over all the vaults only to find that the goblins had magically sealed off the tunnels with goblin blood wards and seemingly collapsed them. Voldemort did not have access to the wealth in there, though with the wizarding economy now in shambles it wasn't like it mattered much.
Eventually their search had lead them back to Gringotts where Ron's brother Bill had shown them a secret entrance to the tunnels. Bill told them that he was one of three non-goblins who could access the tunnels, as he was ranked highly as a curse-breaker. Though Bill had no idea who the other two people were, Harry, Hermione and Ron were happy to be able to have someplace secure to rest. They had even used the ruins and tunnels beneath Gringotts as a base for a little while.
He remembered exploring those tunnels and vaults with Ron and Hermione. They practiced their ward and curse breaking under Bill's supervision, breaking into now abandoned vaults. In a rare moment of leisure the three of them had even taken the time to find some of the Death Eater vaults and pack huge amounts of their gold into multi-compartment trunks. They took great pleasure in doing so as they broke into the Lestrange, the Carrow, and the Malfoy vaults.
At the moment the gold was relatively useless since for all intents and purposes there was no economy. But Ron liked to joke that when the war was over he would be rich and a hero. However, he never got the chance to be either.
He had perished shortly after pushing Harry out of the way of a spell from Voldemort; taking the spell meant for Harry in his back. The spell took Ron down and he was swarmed by demons.
Occasionally Harry wondered if Ron would make the same decision today, given what he'd become.