118 hours after starting to play the game, and after conquering 29 Shards, Ul Dagan conquered me.

Guess I should have kept it at Beginner instead of raising to Skilled a few shards back.

Actually, now that I think about it, I should have really used the "Revert to last turn" ability when my main warrior got killed... But I hadn't used it for so long I forgot all about it...

Look for gameX.bkc or gameX.bka files in your save directory.
Rename one of them to gameX.cmp
X is equal to your profile number-1. IE first profile have game0.bkc, second game1.bkc
These are backups of your shard game (bkc) and astral save (bka)

Thanks, but I don't want to cheat - the game afterwards would feel too hollow.

What a spirit!

Happened to me,too in my first campaign! Pretty amazing feeling, huh? Games don't usually do that. Anyway, now I am am near the end of my third campaign. Will start my fourth soon with the new horizons mod.

I'm used to playing roguelikes with permadeath. I think my record for losing had been a character I put 50 hours into - but I guess records are meant to be broken :). It is annoying that the very first shard I lost had been my own - but it was also my first time against a Master difficulty opponent. I guess he was annoyed that I had just defeated him on a Large shard the previous round (barely... damn his Monster Guard which made me spend over 1000 gold to take each province, but at least he couldn't get past my Monster Guard either, though it also cost me a ton of gold to maintain.....) , and called him a liar to his face before that :)

Started a new campaign on Skilled, thinking to switch to Beginner when it gets hairy, but accidentally hit End Turn on the Astral after the second shard. This may end up costing me, so just gonna start over - not too much progress lost this time.

BTW I hear there is a "turn limit" that depends on your chosen difficulty - is it decided by the lowest difficulty you ever chose in the campaign or what?

I think I'm going to take this lesson to heart - in Defense missions, ALWAYS revert time if your main hero is defeated, unless his sacrifice accomplished something necessary. Lost glory is nothing compared to the potential alternative. Even if he dies in the first few turns of the shard, that's lost time which might turn out to be critical later on.

You could also go back to the astral when you see you'll soon lose (for example when your fortress is sieged), no?
The cost (-2 to your energy income) is quite ridiculous compared with the risk of losing the whole campaign.
Unless this option is disabled when you're defending your shard? (I used it only once, on a normal shard, so I don't know if it's always available.)