Plane of Magic

Imagine for a moment, that you are a mage on the battlefield of war - maybe you've been trained for battle-casting, or maybe you've not been trained in casting Magic under such pressure. Imagine this situation, and imagine you do not have the magical energy to keep casting, but still you must. Where do you reach for the magic?

— University of Tormyra Lecture

The Plane of Magic is not a place where one goes, but any mage can reach out and touch it, draw on it, and use some of the power of the plane when casting - but only when they have no other means of casting. When a mage reaches and pulls on the plane, rifts and ravines will form, often leading to the mage's demise as much as their foe's.
Scholars has studied Magic and its sources, but are unable to confirm if the plane is the true source of magic, or if all mages have such a final-effort access to a collective well of magic they've unknowingly fed into.

Travel

Physically going to the Plane of Magic requires finding where a mage has left a magical imprint, meaning a rift is the easiest way to start.

It was a moment of desperation, but I did it. I won, but in that moment, I felt like a candle where the wick burned through, but the wax had not melted. I took decades to recover, and even now, a century later, I am half the mage I was before that moment when I felt I pulled from that never-ending ocean of magical power.

— Surviving Mage, elf, age 176

Exploration

Attempts to go to the Plane of Magic have been met with failure - no explorers have ever returned after the initial "day" of exploration, despite multiple attempts. Attempts to rescue the explorers have also failed with the would-be rescuers either being lost, or returning without their targets and seeming to be "not all here anymore" and missing memories.
Reports describe the absence of days and nights and instead a constant presence of light coming from below to contrast with the clouded and unbroken sky. There floating island to stand, walk, and camp on, though the islands start to sink after a uncertain period of time regardless of anything or anyone being on the land.
Some scholars believe the light is the sun, though the blue-hued light doesn't match the light of the sun as seen on Corive. Other scholars suggest the light is from Blue Moon, and that the source of all magic is the moon - though the surface is measured based on the horizon of the plane to be smaller than the moon, leading to debates over the size and distance of the moons and the theory generally disregarding the lack of recognizable features of the moon being on the plane.

The sky is a distant dark cloudy night dancing with deep violet and reds. There are no stars, there are no moons. If I had not been granted permission to go to the Undermountains, I would believe we were in the realm of the Dwarves until looking at the land.

The land fragments we walk on are like solid water - but not ice as there is no crystaline flakes despite the clear-like watery-blue coloring. The ground is solid but rippled, but also translucent into other solid-water-land below. Between the fragments, air moves as if a liquid.

We intend to climb or dive as the case may be, further down in a few hours of the hourglass. There is no sun to watch for time except below us is the contant glow. Already, we can report our magical abilities are improved beyond even our best estimations, though the reactions between spells also appears to have exponentially intensified. Basic and common spells that were aimed between the land appear to intensify as they follow their trajectory and out of sight into the blinding not-sun.

One of the scouts went ahead, but returned confused, as if they had forgotten why they were scouting - or what scouting was.

- initial and final report from an exploration team

Show spoiler

Corive is a Hollow Planet where there is a spinning core at the center pulling everything in. This core is the Plane of Magic and going "Down" allows the person to gain more power, but at the cost of forgetting or losing parts of what makes them an individual. I honestly have no idea yet how to convey the idea it's the planet core. That may have to wait for me to write Future Corive