AP's got a lot of good info in there - thanks for that Alex! But I gotta dissent on the Miasma - not only is it a lot of fun, but it seems to be pretty useful in MP.
The +% income is nice - it cancels out the heat scale gold loss, and a slightly warmer is better with C'tis anyway. Especially with the season's shifting the weather around some anyway, at least +2 heat for C'tis is most likely the real sweet spot for them lizards.
Sure Marshmasters are -1 death then the Sauromancer, but the +1 nature +1 water gives them so many more uses. Can't reanimate the dead as effortlessly, but they will get quickness guaranteed, any of them can cast Foul Vapours with an extra gem (and now C'tis is 50% posion immune, so this is usable right away in large battles), and you start with the Summon Bog Beasts spell (as soon as you get a +2 water MM, start cranking them out - only 5 nature gems for 3 of them with only 1 mage turn used). MMs with 3 nature are excellent for Relief/Charm etc without items, so mid game your potential for never-tiring magic Users is really high. And better site searching to boot, so I really prefer MMs over SMs.
The disease effect is not going to wipe out whole armies in a turn, but it will hurt quickly and make them think about it twice right away
If you have the choice of attacking C'tis knowing that your troops are going to get diseased or moving against someone else, I imagine most will go for the other. Couple Miasma with a nice defence castle, and sitting on top of an enemy castle is suicidal so any stall will be extra costly. Just the risk of a supercombatant pretender getting diseased early will likely stop some unwanted visits...
Also, the disease effect is pretty potent against the indies - in a province in your dominion, by turn 10 they will be significantly weakened, and mages/priests with lesser HP are the first to go. The softening up of any underwater defences is very handy - even the Amber clan will disease and die and then you can take them with minor losses. By turn 20 any unclaimed indies nearby will likely be decimated by the disease, and you can take them with a token force. In a strong indie game with a good starting dominion, the disease effect will probably be killing off troops faster than you are, and Miasma would be a huge edge in sparsely populated, low resource, high indy games.
Not being able to use most indies is a big drawback, but you don't need too many anyway. With your increased Nature site finding, you should be one of the big contenders for Gift of Health, so that will largely make the disease effect moot. The only real indies C'tis needs are mages (which are rare anyway) and scouts (which are cheap and mainly used outside your dominion), so it isn't too bad.
I need to try it more in larger MP games, but it seems all good to me!