So you wanted to play a cleric... but the only god in town is a dark and terrible deity set on devouring all life, light, and magic in the world. The rest of the party's not on board with this agenda. So what's a cleric to do?
Heresy, I tell you!
The heretic legate brings a lot to a Midnight party. Healing magic's great, but an air of legitimacy is even better. Of course, like all Midnight characters, you still have to be careful, because if your heresy is discovered, you and all of your companions are in for a very bad time...
Unfortunately, more historical heresies are over theological points that wouldn't really be all that interesting in a D&D game, or which wouldn't make any sense for an evil religion. Slightly more promising are the regional heresies like the Donatists. In Midnight I could see a plausible heresy regarding the doctrinal correctness of the orcish wise women (I apologize for failing to remember how to spell urasatch kuradeen; I don't have the Midnight book available at the moment). The Church of Shadow naturally has no interest in yielding spiritual authority to the orcs, and so having studied the oral tradition of the wise women and decided that it is superior would constitute a minor heresy liable to improve one's relations with the orcs (they are, after all, Izrador's chosen people). Likewise, Sammael and the Order of Truth (or Light, or Shadow, or something?) present a plausible pair of targets for subversion. Maybe some bloodthirsty Dornish cult of Izrador left behind runestones across the North with a syncretic doctrine fusing ancestor worship and undeath with the worship of the Shadow. Their teachings were eliminated from canon by the Order, but continue to appeal to Dornish legates. Individual legates who received (or claim to have received) divine inspiration from Izrador might also challenge the Order's orthodoxy.
Aspect-based heresies might be interesting; sects that emphasize one part of Izrador as more holy than the others, or supreme / dominant. Unfortunately, he's an awfully monolithic deity, and his domains are fairly bland (that I recall). A legate venerating the chaotic transmuter-nature of Izrador, as evidenced by his miraculous transformation of dwarves into orcs and the warping of the Highland Imps, would be a fun and very heretical character unlikely to score any points with the religious establishment, though.
Racial heresies are also tempting. The writings of the Mad Dwarf might reconcile the Shadow and traditional dwarven beliefs beautifully, and so tempt dwarves to the worship of darkness. If one accepts the existence of the Traitor Dwarves (Black Iron Dwarves? I forget their name), one can only assume that such a heresy might find fertile ground there. Likewise, an elven heresy that blends nature worship with Izrador (as a 'world-spirit of darkness') might find purchase in the woods, or worse, bring otherwise Shadow-fearing human legates to the practice of elven sorcery. Such a heresy might even be secretly supported by Ardherin, as such heretical sorcerer-legates could provide cover and breathing room for his apprentices.
We're starting to get in to 40k-grade heresies here, which suggests another. In 40k, unorthodox inquisitors have been known to strike bargains with or even employ xenos. A heretical legate might find it justifiable to employ the fey, as they are useful, might even be convertible to the One True Faith, and are conveniently easy to dispose of when necessary. This is, of course, quite useful for an adventuring party whose players desire to be other species. Unfortunately, it's also still likely to get everyone killed, just slightly more slowly than would normally apply to a party containing fey, at least while under domain level.
A domain-level legate (bishop) could get up to all kinds of fun heresy, though. I find the image of a sect of legates who believe that protecting their flock from rampaging orcs, issuing coinage, and teaching writing for accounts-keeping in order to improve the economy and ultimately strengthen Izrador's war effort being branded heretics amusing. The Propsperist Heresy, if you will. This is also the point where you could start getting away with providing sanctuary to sorcerers and fey, or establish a heretic stronghold (like that one in my Crusader Kings game that keeps revolting... damn Cathars). A heretic bishop would likewise make a convenient patron for a heretic legate and his party, provided they were of similar heresies.
Of course, where there are heresies, there are inquisitions...