I've had or seen several people asking why it is in the Dark Albion setting that the Clerics, with all their power as a single coherent order and all their magic, don't stop the Frogmen? After all, the Cleric's magic and military structure is notable; and their miraculous power can often over-rule the chaos-based magic of the Frogs. Even if we assume the Frogs are too powerful now, why didn't the Clerics stop the Frogs from taking over Frankland in the first place??
(From an Anglish illustration: "Frogmen speaking through the mouths of human heretics":)
To address the first question first: the Frogmen have too much power for the Clerics by themselves to deal with, by a longshot. Consider logistics: the Frogmen control a vast kingdom, and even though they are a minority in their own lands, they are assisted by a relatively loyal cadre of chaos-worshiping humans who know they would be burnt at the stake should the Frogmen fall. These act as the soldiers and overseers of the large peasant population. The Clerical order, on the other hand, does not have authority to control armies directly, but must operate with the kingdoms that they find themselves located in. Its not enough that the clerics might want a holy war against the Frog menace, they would need to convince countries like Albion, Burgundy, Lorraine, Arcadia or Iberia to work together on this.
Second, while clerical magic can be very impressive, Clerics are only a tiny percentage of the population. There are perhaps barely 5000 clerics in all of Albion at any given time; this figure includes clerics who are still children/novices, elderly clerics, the infirm, clerics who run the miraculous sites (like the healing waters of Bath), etc. Only about 1 in 1000 people born are gifted to be clerics (there being about 5 million people in Albion, total), and they are better suited as elite troops (or sometimes, as commanders) than to serve as a rank and file army. The Frogmen are much more than this, there's about 420000 Frogmen ruling over a population of 13 million humans. They have magic items they can use themselves or pass on to trusted humans. They can call on their dark gods to grant powers and mutations. They send out agents to corrupt or divide those kingdoms that oppose them (depending on what you want for your campaign, Queen Margaret may be one of them!). There are plenty of reasons why the Frogmen, having now succeeded, may be very difficult to stamp out.
A perhaps much more interesting question is: why did clerics fail to find and stop the corruption of the Frankish Kingdom in the first place?
There were several reasons for this: first, you have to understand the situation of Clerics. They are an order, within the Church. I mentioned above that the Clerics are only 1 in 1000 humans. Compare this to the clergy (priests, monks, and nuns), which occupies about 1 in 50 adults at this time. Additionally, Clerics are independent of the standard ecclesiastical hierarchy; they govern themselves and are answerable only to the Pontifex. This has certain advantages, in that they can not be manipulated or abused by corrupt priests or bishops, but it also has the disadvantage that their absence from the standard hierarchy makes the latter vulnerable to corruption. Clerics operate in small groups, or are assigned individually to very important nobles, or to a bishop, or work as inquistors, or fight with armies (all these aside from those cleric-administrators that run the clerical houses). That is to say, they are usually pretty spread out. It is thus potentially easy for those corrupted to keep clerics 'in the dark', or if necessary to get rid of individual clerics who find out the truth.
Second, the tactics the Frogmen used: they sent out human agents first, to gradually corrupt the nobility into decadence and sin, while corrupting part of the Church into the Cathari heresy. The latter in particular distracted the Clerics (and other Frankland nobles who were devout to Law), who saw a visible enemy (the heretics) and went off to fight them, losing sight of the more subtle corruption of the aristocracy. In the end the Frankish armies won the battle against the Cathari, but lost the war against the Frogs. Their tired armies were betrayed by some of their own commanders when the Frogmen finally started streaming out of the Paris Swamp. Sects of human chaos-worshipers emerged at the same time all over Frankland.
The Clerics had managed to catch a number of Cults and conspiracies, and to oppose the major heresy (Catharism) before their eyes, but they failed to uncover the masterminds behind it all. They had, until that time, assumed the Frogmen to be dangerous chaos humanoids in the same way Anglishmen view goblins: scary and dangerous but more likely to eat humans than to try to use subtlety to corrupt them. It hit the clerics completely out of left field, along with all the rest of the lands of the Sun.
(they used to look like this:)
(until they suddenly became something like this:)
Finally, a note from the point of view of mechanics: remember that while Clerics have a lot of social advantage in the setting, the Dark Albion setting is generally a low-level world. While Clerics, being (most of them) dedicated to a life of opposing Chaos, tend on average to have more experience than most other people in the world, the average Cleric is still only level 1-4. The greatest, most advanced clerics (the Commanders and Supreme Commanders of the Order) are only level 9-14. So we're talking maybe one to three clerics of level 9+ PER COUNTRY. Someone might ask "why doesn't Commune just solve everything"? The answer is that there's likely only the tiniest number of Clerics alive at any given time who even have that power. As great as the Clerics may be, they're far from omnipotent, nor are they immune to being fooled or betrayed.
Currently Smoking: Lorenzetti Solitario Egg + Brebbia No.8