Vampire

(Armor -2, Health 28 to 154, Damage 20-50 + Spell + Disease + Vampirism)

Likes to sling fuzz-balls at you (Sleep, I think) and then close for melee. They're all female, they hit slowly and don't hit very well, but a real owie when they do connect. Their biggest weapon is their chance of infecting you with vampirism AND other diseases. Fortunately you shouldn’t run into them until you've got the levels to handle yourself. Treasure is money and lots of ingredients. Occasionally they'll have miscellaneous items like a holy dagger or holy tomes (don't ask me why - this is one of those illogical logic things) or a weapon.

Vampire Ancient

(Armor -5, Health 30 to 170, Damage 20-60 + Spell + Vampirism)

A very nasty character. Unless he's pre-placed, you probably won't run into him until you're well over 10th level. He likes spells that can put a severe hurt on you (Paralysis and Shock are his favorites). He hits VERY HARD and closes very fast. Spell Reflection is a must with this guy and you must have Mithril or better weapons to hit him. Treasure is LOTS of money and ingredients with an occasional weapon or miscellaneous item.

Lich

(Armor -10, Health 30 to 170, Damage 70-100 + Spell)

I very seldom have encountered a lich underwater unless I knocked him there first or he was a quest object, but all things are possible. Essentially he's an undead version of the Spellsword or Nightblade. He hits very hard, though. Spell Reflection is helpful (he likes Shock, Fireball and Wizard Rend), but healing and good weapons are more helpful. He's not too picky about where he slings his spells, so it's not unusual that he will blow himself up while trying to get at you.

Ancient Lich

(Armor -12, Health 30 to 170, Damage 70-100 + Spell):

One tough customer. Heavy-duty spell attack (Shock, Fireball, Toxic Cloud, Lightning, Wizard Rend) and very good melee skills. Not for the faint of heart or low of level. Mithril or better weapons to hit him and he's usually got decent goodies.

Daedra

Daedra come in five varieties (aside from the Daedra Princes you can summon, but can't touch): Daedroth (sometimes called a Lesser Daedra), Daedra Seducer, Frost Daedra, Fire Daedra and Daedra Lord. You must have Mithril or better weapons to hit any of these guys.

Daedroth

(Armor 1, Health 27 to 146, Damage 15-50 + Spell)

Spell attack (usually Silence) plus very good melee skills. Mithril or better to hit them. The Fighers Guild isn't kidding when they say be prepared for a tough fight. Treasure is usually money, but there's another item occasionally.

Daedra Seducer

(Armor 1, Health 28 to 154, Attack 15-50 + Spell)

Likes to get in close, zap you with a spell, change into a female with dragon wings and then beat up on you. She's not quite as tough of a fighter as the Daedroth, but still a rough opponent. If you have Spell Reflection up her spells (usually "Energy Leech") will recharge your fatigue if she fails her save. Treasure is money and an occasional ingredient, but I've picked up a couple of armor pieces and weapons off of them, too.

Frost Daedra

(Armor -5, Health 25 to 130, Damage 50-100 + Spell [Ice Bolt/IceStorm])

Fire Daedra

(Armor 1, Health 26 to 138, Damage 15-50 + Spell [Fireball, Fire Storm])

Ranged spell attack (frost or fire, depending) followed by melee. These guys are hard hitters and have lots of hit points. Treasure is almost always money, though the Fire Daedra will occasionally carry something else.

Daedra Lord

(Armor -10, Health 30 to 170, Damage 70-100 + Spell [Shock, Fireball, Wizard Rend])

Easily as tough as the Ancient Lich. Likes to zap you then close for melee. Treasure is almost always money and ingredients.

Atronachs

(Armor 6, Health 25 to 140, Damage 5-15)

Four major varieties: Flesh, Iron, Ice and Fire, but their stats are the same. The original intent of the atronach seems to have been to mirror the four elements of medieval science: earth, air, fire and water. I don't know why this was abandoned, but the remnant of this idea shows up when you try to soul-trap an atronach. When soul trapped, the atronach changes to its appropriate element:

Iron Atronach becomes an Earth Atronach Ice Atronach becomes a Water Atronach Flesh Atronach becomes an Air Atronach Fire Atronach becomes a Fire Atronach (just to keep you from getting cocky)

Atronachs are medium to light hitters, lots of hits points, and no treasure. From a combat perspective, they're about on a par with the Mummy, but no treasure. They are supposed to generate an aura that damages you according to their type, but this is not an active feature (at least not on my system). Their defense is their resistance to certain types of spells.

Fire-based spells will heal a Fire Atronach Electricity-based spells will heal an Iron Atronach Poison/Acid-based spells will heal a Flesh Atronach Frost-based spells will heal an Ice Atronach.

They aren't immune from a good weapon strike, though. If you want money off these guys, pick their pockets before you kill them.