Unemorin the Cursed

A very long time ago, when there were only elves, dwarfs, lizardmen, dragons with their dragonborn and kobold servants and many races that are extinct now, including wendigos, all of them were on the verge of destruction. A high elf named Umenorin, who possessed no special abilities or talents, believed that their world, not yet called Prethoria, is not the only world in the universe.
This idea has been given voice by individuals all the time, but Umenorin was the most stubborn and fanatical out of them. Despite the absence of any successful attempts of getting to other worlds, he continued to search for the ways to do it. Eventually, he stumbled upon an ancient deity who he later only referred to as a Great One.
The Great One said that he came from the world other than this and that he wants to return back to that world. He said that he could teach Umenorin how to open the door and help him, but both of them would need a lot of power to do so. Umenorin agreed, and soon elves and dwarves alike started to disappear in that area. The number of people who went missing was growing rapidly; numerous monsters and wild animals began to attack the settlements of all races.
One day, a scouting party of high elves stumbled across a camp of kobolds overseeing a host of monstrous animals that have never been seen before. The scouts, along with some reinforcements, attacked the camp and quickly overwhelmed its inhabitants, but soon the situation reversed. Another elf walked out of the forest and headed directly towards them. When the wounded elves in the back lines asked the approaching elf to name himself, they fell on the ground with blood oozing out of every inch of their bodies.
The elves immediately attacked the stranger, but he did not pay any mind to their arrows and spears. The rest of the group suffered the same fate, except for a young druid who managed to turn into a little mouse and escape. Seeing the kobolds tie his friends and comrades, he was too afraid to stay and try to help them and ran for his life.
By that time, almost all races have got the sense of something being wrong. The story about an unkillable elf with kobolds and new monsters serving under him, told by the survivor, was the last straw. The war was declared on whoever that was.
Soon, the armies started to march into the area where most of the reports were coming from. They were attacked by monsters all the time, and monsters seemed to adapt to every race's weaknesses. After suffering heavy losses and many of the groups going missing, a joint army was organized, and even more races have decided to send their forces. They broke through the lands creeping with monsters only to find a whole nation of those monstrous races being lead by the same elf.
The war between the coalition and monsters lasted for many years, and during the fights an elf calling himself Umenorin, servant of the Great One, often made his appearance on a back of the giant flying monster that was bigger than a dragon, raining magic on his enemies from above.
Numerous times Umenorin had wounded so hard that no one could survive it. Yet, he continued to show up again and again. Many even claimed that they have seen him dead and with no breath. He returned even stronger every time. No one even believed that he could be killed any more, and the coalition was slowly losing the war. It would seem the fate of everyone in the world was to end up sacrificed to the Great One or to become a part of the giant figure that the crazy elf was drawing on the earth with the blood of those he had captured.
One day, Umenorin and his hoards crushed one of the opposing armies and make a quick move forward. Before anyone could react, they sacked the capital city of one of the small elven kingdoms. This day, the odds changed once more. When Umenorin sacrificed the Head Priest of Efler, god of pleasure, on his own altar, the god enraged. He cast off the unspoken rule of not intervening directly, and Umenorin and all his monsters inside the temple were crashed as if hit by a giant hammer. Umenorin still did not die for good, but one after another, the gods started to intervene more and more.
The war still caused hundreds of deaths every day, so the coalition came up with a plan. A small group of best warriors, mages and priests sneaked deep into enemy lands, broke through the monstrous hordes and killed Umenorin. He was brought back by the Great One once more, but at that time, the gods made their move. The battle between the Great One and several dozens of gods was so fierce that all monsters and heroes were crushed all together along with the very landscape. The gods won, yet at least seven of them were killed during the battle. Umenorin was never to be seen again, and not even his body was found in the bloody mess that became of the battlefield.