Spellbook Virus
Not so much a new monster as a hazard of the professional Magic-User, possibly some sort of specialized curse or adapted feeblemind spell. Magic-Users who frequently copy scrolls to their tomes may suffer unanticipated consequences as the magical energies from different sources comingle and combine in unexpected fashion. Also, spellbooks, being magical, routinely attempt to maintain themselves against bookworms and decay; occasionally these self-maintenance attempts go astray and cause massive difficulties.
In general, any spellbook remains free from viral or other mishaps for 6d6 months. At the expiration of this “warranty period,” the spellbook falls prey to numerous hardware and magical software difficulties. These mishaps may include but are not limited to:
- Erasure of spell notes (be sure to back up your spell book to scrolls!)
- Corruption of spell data (may trigger “wild surges” due to memorization errors)
- Binding disintegration (may cause loss of pages due to physical binding deterioration, or loss of any familiars/bound spirits)
- Vulnerability to astral predation (increases odds of wandering monsters, but mainly of the “outsider” type, possibly including clairvoyant surveillance of the owner and his or her use of the book)
These difficulties can be solved by using a back-up spellbook or scrolls, but Magic-Users with low Wisdom scores are unlikely to have taken such measures in the recent past. They must make a Save vs. Dragon Breath or lose 1 level as if drained, reflecting the untimely and aggravating loss of knowledge and hard-fought experience.
Rubbery Sludge
Armor Class: 8
Hit Dice: 3
Move: 4″
Attacks: 1 per hour
Damage: 2d6 + Special
No. Appearing: 1d4
Save As: Fighter 8
Morale: 12
Treasure: Nil
Alignment: Neutral
The Rubbery Sludge, like the Grey Ooze and the Ochre Jelly, is a blob-like monster found on I-95 between New York City and Washington DC, and it is believed to be created when misfortune overtakes a cargo-drum of alchemists’ reagents along the roadside. The stench is overpowering, but the Sludge itself is not immediately visible. All horses, wagons, and foot traffic to pass through the Sludge must Save vs. Petrification or become stuck in place for 1d6 hours, attracing the usual gamut of wandering monsters. (Owing to difficulties with leverage, those attempting to rescue others from the Sludge must also Save or become stuck themselves. The Rubbery Sludge may be dissolved by acid but it will burn those stuck within as well.) Those so stuck take 2d6 points of psychic damage, cursed with the knowledge that their comrades-in-arms are even then sojourning into the Caverns of Thracia to plunder the jewels that glitter in the darkness, gaining treasure and XP without you, the inconsiderate swine. Victims must also make a Save vs. Poison to avoid embarssing mishaps owing to a lack of sanitary facilities.
The Mule Abides 
Maybe the difference between arcane and divine magic is that the techniques for putting the divine to work for you are more ancient and require a building-sized focus of worship faithfully tended by a cadre of dedicated and specialized acolytes, where you go to submit your prayers (often written on scrolls passed to the priests, for whom direct access to the deity is a much-sought-after boon) and hope that the response will make sense to you and that you didn’t phrase things in such a way that the deity can misinterpret your intention and fulfill your wish in entirely the wrong way?
Arcane magic, by extension, would be a more recent development of techniques for personal access to the power of the gods. As you may have discovered, however, recourse to the priestly class may still be required when things go wrong.