There was one old man that come to my castle during the first few turns of a shard. Obviously, I can't resurrect the old man because I don't have enough gems since it is so early in the game.

Is the old man related to the key of truth?

Does it mean I can no longer complete the campaign by finding the key of truth? Will the old man come again in future shards?

IIRC he cannot be resurrected, trying gives you some +karma but fails.
I don't quite remember if he is related to key of truth or not, maybe... What did he say?

Look for a province labeled --

It is a very convenient bug that whenever you are told to go to province <insert name here> for plot reason then the game picks a random provice and attempts to rename it to the name you were given in the quest (shards are randomly generated).

However the bug in translation will cause the name to be transformed into "--" so you just look through province list and if one has that, conquer and explore it (cleaning each area in it) until something plotty happens.

If he gave you a message that disintegrated as soon as you tried to read it (and did not mention any province) then as soon as you finish conquering the shard and return to astral you will find it "disintregrated" into the astral plane where you have it and are able to read it (IIRC it gives a province name containing a key of truth in another shard, you will not know which until you actually try to conquer said shard)