The years after the demon overlord Vissiluth The nether worlds thrived. Till one day Zanzamar saw a portal open in his home realm. Realizing that another world was out of equilibrium he got his carpet ready to go through. That's when he realized the black beasts coming out of the portal going after the mana and eating it. Then he begun using fireballs on the wolf like beasts going through them like he went through the skeletal archers with a basic spell. That's when bigger beasts came through scorpions, giant birds, and elephants. That's when he begun using his lightning spell and meteors to thin the group. There were only 20 left consisting of 1 bird 12 elephants and 7 scorpions which he used doomsday on while flying low skillfully between them all and dodging their attacks, when doomsday went off they all died. That's when he decided to set up magic triggers near the portal to make volcanoes then a crater after 20 seconds go off under whatever went through to keep them out of his realms.
After setting up his defenses he went through the portal to explore the new realm he was in. He saw he was surrounded by a forest near a city and more of the black beasts only consisting of wolves and bears which he then begun using earthquakes and crater spells to trap them and finish them off in mass with meteors. He then set up a castle which leveled off the ground and collected the mana they dropped in this realm unlike his. There was a village near his castle that he assisted and became the lord and protector of over. He trained 4 of the villagers to use magic like he does and they made small fortifications of castles that can cast fire spells from the towers then after the year was over they were skilled enough to have level 4 lightning towers. The defenders were set up in the north, south, east, and west side of the village training others to do basic fireballs and weapons training to defend the 5 castles Zanzamars was in the middle of the village. After 1 year he then got a stable level 7 castle with lightning towers as well while checking out the city of vale.
When Zanzamar went to vale the first time he was shocked at the technology and lack of magic in this realm and the use of mana which needs to be possessed in order to form mana balls or dissipate. He was flying on the carpet at night when he noticed a robbery going on and saw a girl with a scythe and a firearm in one. He then decided to assist by using lightning killing all the thugs but not the main thief making the girl and caned thief look up in surprise.
Roman saw his hired help get hit by lightning bolts and killed all in an instant looking up, Roman saw Zanzamar on a carpet that was defying gravity and attempted to shoot him down but only to have his shot reflected back to him which he barely dodged. Ruby while watching this had her jaw almost drop in the surprise of the whole event wondering what weapon the flying guy had and stunned at the deaths around her. Roman escaped and left but not without Ruby trying to stop this on the roof till a huntress showed up taking her and Zanzamar to the police station.
I know this is not the greatest writing but I am willing to let others take over or help in getting good chapters done. This is a crossover of the game that came out on dos by bullfrog called magic carpet (using magic carpet, magic carpet hidden worlds, and magic carpet 2 the nether worlds) and rwby.
I would like feed back on the thought and maybe additional ideas. I know I partalilly destroyed the emerald forest with giant ravines and holes in it which I don't know how to make ozpins or anyone elses reaction of yet.