Rat Monarch
Class Features
REGAL HYGIENE: Rat bites corrode enemies. Gains one stack of Corrosive Strike per level.
REGAL PERKS: Sensitive whiskers detect deals. All shops are scouted. Buying an item regenerates 50% health and mana (may overheal).
REGAL SIZE: Rats are the CORRECT size: Small. Lower maximum health and mana, all items are large.
CONVERSION: The Rat Monarch gains +1 Corrosive Strike for every 80 Conversion Points.
Unlocking
The Rat Monarch requires the Enhanced Edition of Desktop Dungeons, and can be unlocked via the Moving of the Cheese quest in the Demonic Library. Once unlocked they move to the Audit Zepplin.
Strategy
Not mentioned in the Class Features is that rats gain precisely 1 base damage per level as opposed to 5. Whether this is a bug or intentional, this means their individual hits deal little damage, even Popcorn usually takes multiple hits to kill for them. However, stacks of Corrosion don't go away, and while only physical attacks stack more Corrosion the damage applies to both further physical attacks and to 
BURNDAYRAZ.
This lends itself to regen fighting and mage builds, even with the rat's decreased health pool and lack of resistance. Because their base damage is so low, items that increase base damage tend to do very little for their damage output. They are best served by items that increase their durability, allowing them to stack several hits worth of Corrosion then finish enemies off with fireballs that deal increased damage. BURNDAYRAZ will deal full damage, although their low mana pool can be a limiting factor. Corrosive damage also applies to knockback, so the Bear Mace is also a valuable prep item, essentially doubling damage from corrosion when knockback applies.
Bear Mace is probably the ideal Blacksmith preparation, but 
Shield is good at early levels. Perhaps Shield only wins out if you take 
Whurrgarbl, in which case holding on to burn stacks is a priority and Bear Mace becomes a double-edged prep at best. Potion preps are of limited value, likely to be drank or converted too early to help much. Item: Alchemist's Scroll Alchemist's Scroll is a unique way of getting around the inventory limitations within just the first few levels while getting a much-needed health boost. Of special note is that potions take up a full slot and the 
Compression Seal does not work for them. This severely limits potion use, and items that normally take up small slots become full slot items. One of the few small items that may still be preparation-worthy is the 
Balanced Dagger, especially for harder dungeons where it is already difficult to take down higher-level enemies in large numbers.
Elven Boots is a good prep item, increasing both magic resistance and mana. The 
Platemail is possibly better, negating significant amounts of damage from lower level monsters, thus making popcorn monsters easier to use. The 
Dragon Shield can be stronger against high level monsters and some bosses, but is weaker against low level enemies. 
Troll Heart could be a decent preparation, but it's hard to justify the locker space. Just be warned that if you buy the Troll Heart early enough in a run to stack it up, then you're probably not making good use of your Regal Perks skill - and the same goes for Item: Alchemist's Seal Alchemist's Seal, though the latter goes a bit further by eating gold and potions you may have wished to keep. 
Bloody Sigil also is better-than-usual on a Rat, but again it's an early-game item and hard to justify the locker space for it.
There are also some non-tanky options, but these may need to be offset with +2 health booster if used as a preparation. One unusual synergy is the 
Gloves of Midas item, due to the Rat's healing when buying items. This can lead to a larger end-game spike, but you will need to ration an inventory slot for rotating out junk items; also, longer dungeons where gold is already plentiful will not see this item achieve much usefulness. Whurrgarbl greatly accelerates the pace when fighting monsters 0-2 levels above you, allowing you to seamlessly blend between the corrosion-building and killing phases - you should probably prep 
Mystera Annur, though, since BURNDAYRAZ will be the star gylph while 
PISORF will disrupt the burning stacks. A Whurrgarbl run plays out much differently than defensive prep runs, allowing you to smoothly burn through a larger number of similar-level enemies at the expense of your ability to apply corrosion stacks early to select high-level enemies.
Regal Perks provides some map scouting early game, revealing 3 by 3 portions of the map, turning the blackspace transparent without removing it. This frequently reveals other features, such as the occasional altar, glyphs, and so on without revealing details about them, although the altars can often be identified by closely examining the piety sparkles around them. More importantly, buying items heals both health and mana. Since there are obviously limited shops and gold is limited, this turns shops into another healing resource to save for a late game spike, requiring an empty item slot as well. The Apothecary preparation at the Bazaar can be helpful as it increases the number of shops.
The biggest drawbacks are the limited potion inventory and need to hit a monster many times to stack Corrosive Strike. This combined with popcorn not popping as easily makes it more difficult to preserve blackspace, when the Corrosive Strike lends itself to regen fighting and Burndayraz. Because of this, Extra Mana Boosters are recommended as a preparation, without them they will end up with 11 mana after finding all boosters on the map... preparing more means enough mana to use two fireballs as opposed to one. The Transmutation Seal is a good choice as well, due to their trait of healing from buying items. The Witch is less useful than for other classes thanks to their limited potion inventory, although a Reflex Potion can be helpful in spiking the boss, allowing them to immediately stack Corrosion on the boss twice. Waiting till late in the run is recommended for maximum corrosion stacks.
Glyph wise, BURNDAYRAZ or PISORF is nearly required. Meanwhile, 
BYSSEPS does little to help them - at best, you can use it to remove magic resistance. 
GETINDARE is more useful for stacking dodge, especially if you don't have BURNDAYRAZ competing for the mana yet, or if you are at max mana with no targets to choose from; in some cases, it will be enough to get a last hit off of your corrosion even without the superior damage from BURNDAYRAZ, especially when you consider the three tiles of regeneration. 
HALPMEH is still useful, but cuts into mana for BURNDAYRAZ - the former is useful for building corrosion stacks, but the latter is more efficient when you're ready to go for the kill. 
APHEELSIK has good interplay with BURNDAYRAZ thanks to burning lowering regeneration; if you can poison a monster for more than 5 turns, then you will build up surplus mana for the occasional fireball. 
BLUDTUPOWA is as useful as it is in any other mage build, but your lower health pool will likely make it necessary to use it in conjunction with corrosion stacks. PISORF quickly becomes the most mana-efficient direct damage spell with just a few corrosion stacks, making it highly profitable to line monsters up together. The downside is that you will lose burn stacks, but most builds should cope well with the trade-off. 
CYDSTEPP can be extremely helpful, allowing you to build corrosion stacks on much higher-level enemies despite having a reduced health pool. When you have enough stacks, switch to PISORF or BURNDAYRAZ. 
WEYTWUT is hard to justify putting in your coveted inventory space, but given how picky a Rat needs to be about her fights, you may want to leave this glyph on the ground for later just in case - you don't want your progress blocked by a Level 9 with Curse or Mana Burn. 
LEMMISI seems like a good thing to spam and then convert, and 
WONAFYT can be useful early-to-mid-game with proper corrosion stack management. 
ENDISWAL combines some features of WEYTWUT and CYDSTEPP in a single package, and in less extreme cases than when you'd need CYDSTEPP, it can still function like a poor man's HALPMEH. Finally, 
IMAWAL can reward a dive for big prey in spades, even if you only use the bonus once or twice.
Deity wise, deities that improve survivability or glyph use are recommended. 
Tikki Tooki is nearly useless, as rats need to take a few hits before taking much of anything down. This is a pity, since more Reflex potions would work well with stacking Corrosive Strike quickly. Shenanigans with GETINDARE, pre-stacked corrosion, and a plan for converting out of the religion might be possible, but spending gold for piety would cut into your regeneration. 
Taurog punishes glyph use and so can be challenging to use, but the armor is good. Skullpicker is nearly useless like other base damage increases however, and the hit to mana is a bad deal generally. Also, it can be difficult to farm piety until the late game, given how almost all popcorn will take more than one hit without careful budgeting of corrosion stacks mid-game. Mystera Annur is a great deity. The Rat's base damage is terrible anyway, so her Flames boon is all carrot and no stick for a Rat. Mystic Balance favors CYDSTEPP or BURNDAYRAZ while punishing PISORF. Refreshment can compensate somewhat for the rat's poor inventory size in some circumstances, giving you more options as to whether and when you convert glyphs. Another good deity is 
The Earthmother, as her Vine Form helps the Rat's small health pool and provides some damage reduction, in addition to Clearance providing mana for more fireballs or knockbacks. 
Dracul helps with survivability, his boons granting Lifesteal and increasing resistances, although with an already decreased health pool Sanguine can be more troubling than for most other classes to use. Because of their limited potion space in inventory, 
Jehora Jeheyu may actually be a better deal for them than most other classes, giving them more mileage out of their potions as well as buffing their low health and mana per level. 
Binlor Ironshield is a good choice for resistances, and is a good first god or even altar prep because of the free PISORF glyph. Stone Fist's knockback can deal an additional corrosion proc on the knockback separate from that of the original attack. Because of this and the easy piety gain from destroying walls, Binlor is excellent as part of a conversion strategy. Stone Form is only useful if you're up against magic resistance, but then again, you might be better served by lining up PISORF shots anyway. You might consider buying ENDISWAL if you want to take on high-level physical damage monsters early, but if inventory is a limiting factor, you might do better (and save coveted mana) through calculated use of the Stone Skin boon.
The biggest problem for rats is a low health pool, lack of resistances, and needing to survive hits from enemies to stack their corrosion. They're regen fighters that are squishy, and mages have low mana. Their strength is that enemies can fully heal without losing their corrosion stacks, and against enemies that require a lot of hits to take down anyway such as certain bosses Rats are perhaps the strongest class. If they can take a hit from a high level enemy, they can probably take it down with enough of their own hits because of the corrosion stacks, it's just a matter of whether it is worth the resources spent doing so. It is technically possible for a rat with Cydstep, Burndayraz, and some conversion fodder to take down a level 9 enemy at level 2 or 3, though it is not necessarily recommended since that will consume a lot of blackspace to get that many hits in. Another option is bringing the Balanced Dagger and just hunting enemies the same level in the hopes of using less blackspace to win, though this does take up precious inventory space for them.
