As you say, this is how Green Slime is presented in the DMG (3.5e p77, 3.0 p117). Note that Green Slime is considered a hazard, much like a flow of lava, raging river, or 40' pit would be a hazard.
Green slime is green slime. Does the module say the slime is invisible? That the players can't see it? Is there an illusory wall or darkness spell concealing the ceiling? Exactly what is stopping the players from looking around and seeing that the walls and/or ceiling of this room are entirely covered in thick, wet gobs of outrageously neon green slime? Green slime is meant to attack characters that go running down corridors heedless of obstacles or without light sources, or to restrict their movements. Without magical aid or PC stupidity, it's not subtle. Even if the module says "the PCs don't immediately notice" the slime, that doesn't mean it's sequestered. If the PCs say "I'll check the area for traps" or "is there anything on the floor or ceiling?" then they're going to see it. Sometimes you have to read between the lines.
Also, remember that green slime responds to movement—any movement that might be big enough to be food. It need not be a PC. It need not even be alive. Green slime is not cunning. It doesn't wait for the right time to strike. It drops on the first thing of reasonable size that moves underneath it, so it's unlikely that more than one PC would be affected. It would not wait for the whole party to be below it to drop.
Next, most DMs I've played with rule that the slime has to burn through the armor before it deals damage to PCs. The rules aren't clear, but they do imply that this is what happens. Combining that with the fact that it states that fire destroys it (such as a torch) and it can be scraped off during the first round (such as with a torch or dagger), and you see why it's only CR 4 (the same as an 80' pit, or roughly a 20' spiked pit covered with an illusory wall). Green slime is literally defeated by uttering the phrase "I scrape it off with my dagger". This means that, IMX, in almost every instance a player has been affected by green slime the results have been that the PC's armor and weapon have been damaged or destroyed... and that's about it.
Truly, the difficulty of green slime is getting past it once you know it's there. It's still deadly, but you need to find a way to get a lot of fire on the ceiling (burning hands or produce flame) or get it to drop for you to burn ("what did we do with that orc's body?") or find a way around.
According to Dungeon Master's Guide p.68, conveniently available on the Wizards of the Coast's website as a PDF:
The Far Realm is outside the known multiverse. In fact, it might be an entirely separate universe with its own physical and magical laws. Where stray energies from the Far Realm leak onto another plane, matter is warped into alien shapes that defy understandable geometry and biology. Aberrations such as mind flayers and beholders are either from this plane or shaped by its strange influence.
The entities that abide in the Far Realm itself are too alien for a normal mind to accept without strain. Titanic creatures swim through nothingness there, and unspeakable things whisper awful truths to those who dare listen. For mortals, knowledge of the Far Realm is a struggle of the mind to overcome the boundaries of matter, space, and sanity. Some warlocks embrace this struggle by forming pacts with entities there. Anyone who has seen the Far Realm mutters about eyes, tentacles, and horror.
The Far Realm has no well-known portals, or at least none that are still viable. Ancient elves once opened a vast portal to the Far Realm within a mountain called Firestorm Peak, but their civilization imploded in bloody terror and the portal’s location—even its home world—is long forgotten. Lost portals might still exist, marked by an alien magic that mutates the area around them.
Player's Handbook, p.302, summarizes the same information.
Seeing a creature from the Far Realm can risk an individual's sanity (DMG p. 265).
Githzerai philosophers are aware of the existence of the Far Realm, and it may be connected to the aboleths somehow (MM p. 14).
Some users of wild magic are so afflicted due to contact with the Far Realm. (PHB p. 103).
Some warlocks use the ancient knowledge of beings of the Far Realm. Warlocks of the Great Old One may worship such unfathomable beings. (PHB p. 105).
Powerful, world-shattering beings known as the Elder Evils are speculated to be creatures of the Far Realm (Mordenkainen's Tome of Foes p. 234).
Far Realm lore from earlier editions of the game can also be used in your campaign. However, much about this plane is intentionally left undefined in order to set a mysterious and unsettling atmosphere, so lacking much rigidly defined lore may be to your advantage. You may find it useful to read the works of H.P. Lovecraft for inspiration.
Best Answer
The Greyhawk Wiki has a good article on Keraptis.
The article cites four canon sources:
However, it's likely that the article summarizes all the main lore contained in those sources.
The article also cites other sources from the Greyhawk fan community, but these are likely to be non-canonical.
This answer notes two Adventurer's League season 6 modules from 2017 which mention Keraptis, and aren't currently referenced in the Greyhawk wiki article.