The Plain of Taranis is a basic steppe biome marked by rocky yardangs (areas of wind sculped rock outcroppings), archipelagos of eroded megalithic boulders, low scrublands, scarce water, and excitingly, areas of spongy karst terrain where the footing can completely vanish, dropping unsuspecting travelers into slick walled limestone caverns, many of which are underground lakes or rivers. The Plain of Taranis is marked by scattered thunderstorms that are frequent and intense, with a great deal of cloud to ground lightning.
Yardangs - gardens of wind sculpted rocks, these places are haunted and often littered with bones of creatures great and small who thought to find shelter among the stones.
The Rock Islands - not unlike oceanic islands, the rock islands of the plain harbor life and water in their stony folds. They offer respite from sandy terrain and the tremor sensing creatures that dwell under it, like sand worms and bulettes.
Karst - karsts are formed from rain dissolving through limestone bedrock, creating a myriad of caverns, chasms, underground rivers and lakes, and other rock formations where solid ground might actually be a honeycomb of chambers and crevasses. Like ice, the footing can be treacherous, and the ground can just simply give way into a giant sinkhole, or a tiny pothole. Travelers in a rush risk everything from broken ankles and crippled horses to vanishing into a hole in the earth.
The most outstanding feature of the Plain of Taranis is the constant storms and the threats posed by them. If the wind, slashing rain, and staccato lightning aren't dangerous enough, the plain is home to lightning elementals, who can be found there regularly. These energetic spirits can be found at almost any time, but during the storms, they increase in frequency and much larger elementals can be encountered.
Maduin Soul - also known as greater lightning elementals, Maduin Soul normally manifest as large radiant spheres crackling with internal power. They have a definite core, as well as a 'shield' that surrounds them, almost like a biological cell. They can manifest many lightning and storm based powers. If persuaded, a maduin soul can take on a temporary humanoid form, giving credence to the legends of storm giants. In this form, a Maduin can be a valuable combatant, a powerful ranged damage dealer, or an incredibly fast striker. While this is possible, most who summon a Maduin soul do so to employ them as a guide and a guardian to protect them from other elementals. A caravan protected by a Maduin or three will weave its way through storms, and the Maduins can ward away mundane lightning and storm spirits.
Izo - spirits of the storm, Izo are composite elementals, containing lightning, wind, water, and a smattering of other forces, making them very powerful, even if they are not bright. The Izo pass through the air, crash into the earth, and rip a physical form for themselves from rock and stone. This form bristles and trembles with their raw power, and as such, they assume the forms of powerful animals. Horses, bulls, dragons, great wyrms, bears, and more have been seen ripping themselves from the white rocky soil of Taranis. The Izo are headstrong, stubborn, and staggeringly powerful, easily able to overpower low and middle level heroes.
Fulgolems - A fulgolem is a rare and dangerous thing, created when a lightning elemental destroys a magical statue, and the two are fused into a lightning golem. The Fulgolem is a collection of smashed pieces, holding position to match the original statue, with arcs of lightning dancing between the pieces. It is furious and mad, unable to escape it's physical prison. Fulgolems are quick and dangerous, and thankfully rare.
Astrapi Undead - should a person on the Plain of Taranis be struck instantly dead by a bolt of lightning, there is a chance that their blackened corpse could shudder back to a hideous semblance of life, animated by the elemental force of the lightning that killed them. The Astrapi Undead are marked by the scent of lightning and burnt flesh that follows them. They are lost, mindless, and unlike traditional zombies, they aren't terribly hungry or violent. If attacked they will act like normal zombies, but most are left alone to wander the plains, or are put down to release their trapped spirits.
Also known as the Giant of Light, Taranis is the primordial entity of storms. Thunder is the beat of her heart. Lightning is her gaze. The roar of the storm is her voice. Once, according to the legends, Taranis strode the world with the other primordials, shaping the land and the sky, laughing and playing at games with the others who were before time existed. The Plain that bears her name is thought to have been her home, and the broken boulders and stone ruins that are scattered across the plain are said to be the rubble of her tower.
Where the primordials went no one knows, not even the gods themselves. They only know that the supernal ones created them as stewards of their great creation, and then they left. Or most of them left. With Taranis is is said that she did not leave, but either she was too in love with this world to forsake it for wherever the primordials wents, or that she was slain and that somewhere under the karst her imperishable corpse lies still, creating the endless storms and lightning that frolic across the plain.
There is a lost city at the heart of the plain, full of treasure and wonders from the time of the primordials.
There is a simple and humble tomb in the dead center of the plain, and Taranis lies there. What sort of things could an archmage or necromancer do with the corpse of something powerful enough to make the gods themselves?
There is a powerful relic out on the plain, akin to Zeus's Thunderbolt. It is still working, randomly emitting storms and summoning lightning spirits. If someone were to seize this weapon, they could conquer the entire world.
The Children of Glass call the Plain of Taranis home, but have their own mythos about the plains, and they do not share it with outsiders. They have a number of cities and camps across the plain, and many caravans crossing do so through their cities. The wealth of the Children of Glass is their glasswork and beads, which they generally trade for basic goods and luxury goods.
The Lost Heart of Taranis - the PCs have to search for the dingus in the center of the plain, where the storms are the most intense and almost never cease.
A to B - the PCs have a destination to reach, they just have to cross the plain of Taranis to get there.
Storm in a Bottle - the PCs need lightning magic power, the plain is just the place to get it.