[Nightblade]

I recommend an aggressive attitude. Take all the sea provinces straight away and gain a foothold on the land. Once you've done that make a move against your nearest enemy straight away. Recruit masses of independant human priests. Not just a few but dozens. Position them behind a solid front guard of high-morale high-prot troops (crushers or living statues would be great... but it's better to use, say, heavy infantry, than to risk waiting). Back these up with illithids and/or archers.
Give the army enough wine bags so they can survive in 0 supply provinces.
Send it to attack Ermor's capital. Once you seige that, they'll focus their armies on liberating it. Hold them off while you deploy other armies to take their provinces and destroy their temples.

You are right for this, it seems being aggressive since the very beginning is my only hope to avoid being crushed by never ending undead hordes.
In my previous attempts, i waited too many turns thinking more about researching new magic spells instead of conquering province, and so when i got my armies started i couldn't get a real beachead on the continent, as the undead were numerous enough to throw me back into the sea at any turn.

Having a strong dominion could well be useful against ermor as their dominion is an important weapon for them. I'm not sure how AI ermor designs its dominion. If it has any sense then it will also take a high dominion. This means that it won't suffer much from insanity unless it's taking your provinces, by which point you're already probably doomed.

About this point, in the manual (page 130 i think), it is said that LA R'lyeh dominion kills population, similar to the LA Ermor one.

So if i get a too strong dominion and so finally spread it in some nearby province, will this not in fact be helping Ermor to raise even more undead army with the corpse while creating fewer income for me (adding a difficulty to maintain the illithids troops)

Your freespawn are probably no match for ermor's. Ermor is the king at this type of warfare - you can't hope to crush them with hordes of chaff. I'm not convinced that Rl'yeh is a great choice against Ermor. Ideally you want access to nature magic, affordable priests, decent missile troops (Rl'yeh is ok here) and some solid front line infantry. Being amphibious does help, but in general if you can take the land Ermor is doomed anyway even if it takes a while for you to conquer their undersea provinces.

That said, I think that an undersea nation (atlantis) was the first I beat Ermor with in Dominions1. I remember I was so weakened by the war that the ungrateful land nations slaughtered all my armies and drove me back into the sea - one of the few times the AI has engineered a major victory against me. I still won the game though, of course.
The ashen empire is perhaps the hardest nation toplay against in SP. It took me a long time to be able to beat them, hope you have better luck!

Ermor is more than a challenge for R'lyeh, that's certain, the only time i beat LA Ermor was with LA Marignon and hordes of Inquisitor that were +/- successfull at blocking the undead horde in some "chokepoints", while my very strong dominion was spreading through all the map, winning the game for me.

When taking R'lyeh, at first i thought the dominion spread would be even easier, as the Silent Sea wraparound map allow this kind of strategy from several sides as the sea is all around the main continent .
But in practice, i just forgot that Ermor undead can just go into the sea without a problem, while i have some difficulty in going to the land myself.

So basically from your suggestion i should be heading to conquer the land as soon as possible to have a chance, and ignore the sea until the land from Ermor is done for the count ?

Oh, why did you choose silent seas *wraparound* when playing as Rl'yeh? Doesn't the normal version have more sea provinces?

The normal version has more sea provinces, but it is "closed"
In the case of the R'lyeh vs Ermor it certainly does not make the whole situation easy for me, but i appreciate the feeling of having a round world, and the new strategy possibilities having a non closed map can allow.
Even if in that case, it is allowing Ermor to attack me from more angles in the same time.