I've had a little idea in mind for a fictional setting, where I wanted to create two dialects of an existing language. Basically, it would be the 'languages' of two opposing factions; in this particular case, good and evil. The dialects are called Elysiani and Tartaran, respectively. The dialects are to be of Ancient Greek, though perhaps a little more separated than normal dialects would be.
Ideally, I would have the two languages to be mutually unintelligible - that is, a speaker of Elysiani would not understand most of what it said by a speaker of Tartaran, and vice versa.
So far, my work is mostly on morphology and lexicon, given my very limited understanding of the Greek language, both ancient and modern. Based on what I've done right now, Elysiani is closer to Modern Greek, and Tartaran is closer to Classical Greek, specifically the Argosian dialect. If anyone could provide ideas and/or insight, that would be wonderful.
(And yes, I know the difference between a dialect and a language. It's perfectly possible for two dialects of the same language to be mutually unintelligible though!