[LintMan]

I'm not an expert, but I can (hopefully) give you a decent overview.

First, you'll need to decide which Ermor theme you want to use. I'm going to ignore Broken Empire (BE Ermor), which I haven't played, but which I believe is more standard than the other two. Ashen Empire (AE Ermor) is probably the most popular, followed by Soul Gate (SG Ermor). These two themes (AE and SG) have strong "killer dominion", which means that populations in provinces with positive dominion die off rather quickly - much more quickly than the death scales do. This dominion also "auto-summons" units in all provinces you have dominion in. The higher your dominion, the more/better units appear. You get these units in place of recruitable national units - you can only recruit independent troops (but you really don't want to do that and can't aford it anyway). Thus a high dominion is critical for Ermor.

The killer dominion also means that your provinces end up with 0 (or close to it) population in a matter of turns. These provinces are essentially permanently dead, and generate no gold, production or supplies, and *that* has several important effects.
1) The province is now utterly, *permanently* useless to your enemies and a barrier to them because their armies will starve there unless they provide them with magic supply items. So even if they capture a dead province from you, it serves them little good.
2) If your domain spreads into enemy territory, it's hurting them even if you're not attacking them. This makes you a big threat and a common enemy when you play MP games. The AI isn't so smart about this though and doesn't handle this well.
3) You (as Ermor) have very very little income and employing mercenaries or food-eating summoned creatures has the same starvation problems as your enemies, unless you can keep those units ahead of your killer domain (ie: invading enemy territory.

AE/SG Ermor have no recruitable units/leaders, but they do have some special summonable ones. You need a strong death gem income to be able to summon all the units you need - death gems are like gold to Ermor, and most actual gold will need to go to build temples and castles. You'll need to build numerous temples to spread your dominion, because Ermor's unholy priests cannot preach at all to raise dominion that way.

The main differences between AE and SG are that AE gets hordes of mostly low-end undead: ie: longdead, longdead horsemen. SG Ermor gets less units, but they are (almost?) all ethereal which means they are a bit harder to kill, and some have paralysis/strength draining ability. They also have somewhat different special summonable units.

A pretender design suggestion:
Turmoil 3 - Dead pop means no income anyway!
Sloth 3 - Dead pop means no production anyway!
Cold 3 - All your units are cold-resistant!
Luck 3 - Luck events provide some gold, and bad luck combined with turmoil is a bad combo.
Magic 3 - Faster research

This gives you lots of points to spend on stuff like high starting dominion (I suggest 8-9 to start) and lots of magic for your pretender (and/or a more expensive chassis).

You can aim to make your pretender an SC that can conquer provinces (and armies) all by itself, or go for a pretender with broad magic and strong research capabilities.

I suspect that in the time it took me to write this, many other people will have posted their own tips, so my apologies if this is repeating what others may have said!