"Char? You want me to tell you about Char? Oh man, I still wake up screaming from the nightmares I have about that pit. A thick layer of ash covers everything, and there's this eerie red glow that comes from the magma that seeps up through the surface of the planet. One wrong step and you find yourself wading in a pool of hot lava. You ever hear the scream of a man as he is being burned alive by lava, Major? It's not something you soon forget, let me tell you. And if the infernal planet doesn't get you, there's always the damned Zerg that'll try and finish the job. Those devils were everywhere, and they killed a whole lot of my buddies in ways I don't want to remember. Char was a hellhole, alright, and I rather have you kill me than send me back there again."
- Debriefing of UED Marine 385DTA47631
This is from the mouth of a Soldier who stepped foot on this deadly planet.  This burning planet is bombarded with cosmic radiation in a binary star system.  Char lies within the dreaded Zerg Quarantine Zone and is considered to be heavily infested by the creatures.  The acrid air has a high oxygen and sulfur content and is laced with ash, and the average temperature is 48 degrees centigrade with very low humidity.  An unprotected human may survive on the planet, however uncomfortably. Despite the conditions small pockets of liquid water exist on the surface, and precipitation occurs.
Char has thick layers of ash which can wreak havoc on sensitive equipment and can also cover hazards such as magma pools. Clouds of ash form which are thick enough to block the view of the stars, making navigation difficult. In addition, fluctuating magnetic fields render magnetic compasses unreliable.
The magma, driven by powerful tectonic activity, often seeps to the surface.  The harsh tectonic conditions can cause horrifying firestorms which unrelentingly burn across the plains; the tectonic activity can result in pyroclastic flows traveling down mountains at speeds of over 100 miles per hour and with temperatures in excess of 800 centigrade (1470 °F). Temperatures are sometimes so intense that entire sections of its surface become liquefied.
In short, this deadly landscape is where the Zerg call home.  It is more hostile than any place seen before and since.
TL;DR:
- High tempretures mean that fire attacks get +1BAP and water attacks get -1BAP.
- Bug, Poison, Rock, and Flying types are more mobile in this environment, decreasing the BAP of all incoming Super-Effective attacks by 1.
- The ash-filled air prevents the use of moves and abilities that summon Hail, Sun, and Rain without a Weather Rock, even then the weather lasts only for 2 Rounds.
- The intense cosmic radiation lowers the accuracy of all Psychic, Steel, Ghost, and Fairy attacks by a flat 10 due to the interference it causes.
-  Every two rounds, there is a 15% chance that a Lava Flow happens.  When this happens, all Pokemon take damage from a R4 Lava Plume;  This will hit through Substitute, Double Team, and any P/E moves without affecting them regularly.
- The battle will occasionally cause a Zergling to appear;  At the end of every round, there is a 65% that a 20HP Rattata with a full Lv-Up movepool and a neutral will appear.  Each Rattata will use a single (1) random attack against a random foe;  KOing this Rattata will NOT reward any KOC or KO bonus to the ref; Zerglings (summoned Rattata) are immune to Lava Flows.