ELEVATOR PITCH:
In Norse mythology, Yggdrasil is the multiversal great tree that is linked to all dimensions of reality. In Heroes of Yggdrasil, the One Tree comes to life in a unique and unexpected way: it’s a living planet floating in space upon which our adventure takes place. It’s so huge that each leaf is the equivalent of a region hundreds of miles across and each branch is a country. Despite the exquisite natural and magical balance of life on Yggdrasil, great forces seek to destroy the very foundation of life. Will heroes rise up to prevent catastrophe and the destruction of the One Tree itself?
HISTORY
Yggdrasil was the first living being in the universe. A non-sentient concentration of life, it grew sparked by magic infused in the birth of the universe. Over millennia, life forms have evolved on Yggdrasil just as it has on many planets in the universe. The pantheon of Gods from classic Norse mythology arose from the magic of the tree, dedicating themselves to different aspects of existence—from wind and water to storms and battle, knowledge and wisdom. Worship of these deities as well as prominent Druidic-lead worship of Yggdrasil itself is widespread across the Tree.
Over time, nations have arisen. Some branches and leaves are wild and untamed, while others have become small states or countries. Diverse forms of political rule exist from tyrannical to egalitarian democracies. Rings of asteroids circle the tree and some mages have specialized in magic that allows teleportation and survival in these harsh environments. Those nations that are expert miners have become the wealthiest and most powerful on the tree. Dwarven, Kobold and human nations have become dominant powers but also tribes of Dragonborn have positioned themselves as global players.
Two secret organizations as well have gained power over the years and compete with other communities. The cult of Loki, which is known as the Horns of Loki, is a mix of rogues and clerics who set out to undermine organized society and cause chaos. They also have long-term plans to trigger Ragnarok to gain revenge for their God. A second community that vies for power are the hags. Their society is called The Hagafia, and they have perfected the art of transformation, illusion and disguise allowing them to infiltrate all major nations around the tree. With well-placed agents everywhere, and magical communication methods via cauldrons, they tip the balance of power in their favor and bide their time.
And lastly a third power endangers all life on Yggdrasil. A demon from the plane of the Abyss known as Orcus has conquered a dimension known as Mechanus where Constructs are built. He has established a corrupted process whereby demons and constructs are combined using necromantic magic, and has been carefully unleashing them via portals onto Yggdrasil in furious jealousy at this source of life. The Demon-Constructs carry toxic demon blood which they have been using to poison farms, rivers and seas. And he has dark plans to escalate the destruction.
SETTING
The One Tree is the source of all life. As it floats in space, it circles around a sun—topmost point closest. As a result, the branches closest to the top are tropical, the middle zone is temperate (the majority of humanoid races live there) while the roots of the tree are covered in miles of thick ice—comets and frozen ice chunks from space are embedded in the roots which absorb the water. Water rises through tree veins which are so big they stream like rivers (intelligent species have torn them open) and collect in great seas.
Dirt has built up on the surface of leaves over millennia as particles from leaves and bark fall to form topsoil where plants grow. Caverns can develop hundreds of feet down into the earth and leaves can be up to 1 mile deep in parts.
Gondolas on pulleys are used to travel between branches; while vehicles with hook wheels and swing seats can travel on the trunk itself. The flying races are experts at traveling from leaf to leaf so they can be hired as navigator to lead a group using parachutes or gliders.
Leaves are identified by a name plus number, the number indicating how far from the trunk they are—lower numbers being closest to the trunk. While lower branches have less light exposure overall, across the whole tree there is no night or day. Due to a slight wobble in the tree rotation, upper leaves can shade lower leaves randomly. At any given half-hour interval, the gamemaster rolls on a random table to determine the degree of light/darkness outside. Despite this, time is broken into eight-hour increments for a day of 24 hours because the tree rotates every 24 hours.
Up to this point, as the worldbuilder and gamemaster, I have created an outline for the populations on nine branches and 19 leaves. I have designed maps for six leaves as well.
PLOT LINES
There are two master fronts that players will need to confront to prevent destruction of the Tree. Both have a series of events that grow in danger and impact over time as players grow more powerful. The fronts will also intertwine and give players the opportunity to make independent decisions on how and where to address the dangers.
Players begin their lives on the leaf known as Bindelstaff 8. Either in the city of Bindelstaff or in the surrounding countryside or a farm. Gamemasters should work with PCs to create a backstory relationships between each of the characters as well as connections to the leaf Bindelstaff 8 so that there is a personal impact on the primary events that follow. Also, some of them, but not all, should have some spiritual connection to one of the Norse gods, to the Hagafia or to one of the other primary factions. The backstory of the players will invest them in the importance of the events that occur.
PRIMARY FRONT ONE
The Demon Constructs poison the tree with toxic demon blood.
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Knife-armed Demon Constructs drop onto farms on Bindelstaff from above, killing farmers and poisoning the land entirely
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Ruins on a leaf above Bindelstaff--Serando 8--are revealed to be a staging ground
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Clues lead to the discovery that an asteroid is where Demon-Constructs are being prepared for attack; the players must teleport to the asteroid and destroy the bizarre demon hybrids who took over this mining site from the Kobold nation
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Rivers and seas are becoming polluted as demon-constructs poison them
Sea creatures are mutating and attacking boats and fisherfolk
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Food shortages are becoming wide-spread leading to riots and starvation
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More powerful demon-constructs have been burrowing through leaves, stems and branches attempting to undermine physical stability
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One leaf stem has been hacked through and falls off its branch crushing an entire country of people
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Players will eventually track down these monsters and destroy their source but then tree-wide water supplies begin to shrink
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Players will track the source to the roots of Yggdrasil where an immense Corrupted Purple Worm/Demon-Construct hybrid is chewing through the roots.
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Finally, once the players have reached their highest level of power attainable, they must travel to Mechanus itself and evict the demon overlord Orcus
PRIMARY FRONT TWO
Squash the Horns of Loki in their efforts to sow chaos and bring about Ragnarok
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Whichever player has the strongest connection to a Norse god is betrayed by a leader in their temple who was secretly aligned with the Horns of Loki; this betrayer becomes a nemesis NPC until the players can finally defeat them; the GM should continue to level up this nemesis as the PCs grow to keep them challenging
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Key players might be imprisoned and set for ritual sacrifice by their nemesis—a rescue must be staged
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Players uncover a connection between a thieves guild and the horns of Loki
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25 years ago there was a major war that impacted many branches: the Ogre War; their nemesis or other leaders of the Horns of Loki are plotting to trigger a second Ogre War
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A Frost Giant hired as a mercenary by the Horns is meeting with large ogre tribes to encourage them to attack the human nation of Elgoreth (Bindelstaff is part of Elgoreth)
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Plots within plots: the Horns have a secondary secret purpose; the second Ogre War is an excuse to get their hands on the great Jotunn Horn kept in a vault in the human capitol
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The Jotunn Horn if blown will awaken a 300 ft tall Jotunn who has been sleeping for centuries
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After it awakens and begins to ravage the countryside, the Horns will now have access to the cave blocked by its sleeping form—this cave is a path to the Underworld where they will travel to free the great wolf Fenrir and thus trigger the beginning of Ragnarok
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Can the players interfere with their plans?
TAGS
Fantasy, magic, adventure, heroes, danger, intrigue, combat, destruction, demons, epic
I have also designed numerous Encounter Maps. Below are some 3D images of just a few of the encounters I've built so far.
Wood Elf Ruins
Chapel in secret Temple of Loki
Surprise attack of plant Blights 
Fight with the Lake Troll
Scenes for the Asteroid assault
Part of an Elven Treehouse
Flying Warship
Some of the NPCs and monster that have appeared in my world:
Andrich Fjorismod, the Frost Giant mercenary
Kobold Chief Engineer Ragna Moonscar
King Kobold
Dirrad the Aarakocra messenger
Due Watchu the bodiless Warforged
Princess Pungy Roofter the Orc
Auntie Tattered Nann, leader of the Hagafia (hag mafia)
Lake Troll (homebrew monster)
Slazekunk (demon-construct hybrid, homebrew monster)
Clockwork Bulezau (demon-construct hybrid homebrew monster)
Birthing Demon and Acid Blood Baby demon
) Some 9+/10s for me include: Cowboy Bebop, Angel Beats!, Log Horizon S1, Dr. Stone S1, and Death Note (this last one is just insanely nostalgic and so easy to watch for anime noobs like I am, but also was in 2009 when I first saw it)