There are extra "instances" which may (or may not) appear in any one section of the game. I check the whole map before actually going to the "goal" for this area just in case one of these shows up. It's a great chance for extra experience and goodies -- and in some of them you will "rescue" a person who turns out to be a specialized gear vendor. You can buy rare quality items from these vendors. Each of these mini-dungeons will have a stone at the end that teleports you back to where you entered.
WITCH DOCTOR - At level 5 you get flaming bats. Later on, you get a pack of undead dog-kinda thingamabobs.
Certain areas trigger a random event. Some areas (cellars, etc) have merchants there (usually offering a rare item for sale in exchange for having saved them from monsters.)
Southern Highlands: Cave of the Moon Clan (2 levels)
* Abandoned Servant House -
Northern Highlands: Watch Tower
Black Canyon Mines:
* Deserted Cellar may have a rare level boss. Once defeated, this allows you access to the Tunnels of the Rockworm, which is a single level dungeon
* The Breeched Keep:
Mehtan the Necromancer - one of the good guys. Help him defeat the bad guys (often with a side order of whatever's wandering around the place at the time.)
* Hadi's Claim Mines: One room. One monster. You know what to do with it.
THE CURSED HOLD
* Free Queen Asylla's servants
THE OLD RUINS
Dark Cellar - monsters. You know what to do.
Equipping the Rogue:
He'll need two-handed crossbows or regular bows. After level 18, special "relic" items (they look like a pair of dice) drop for him. Pick the ones with dexterity. Gem his weapons with RED (strength) gems, but gem his rings with GREEN (dexterity) gems.
OLD TRISTAM ROAD
The Cave Under The Well - monsters. Have fun.
TRAILING THE COVEN
Make sure you're well geared for the "approach the Kharza barricade" section. There will be wave after wave of the goat men.
* The Lyceum - if this appears, you'll have a quest to explore it. The same architect who worked on the crypts, etc, was apparently responsible for this structure.
ROAD TO ALCARNUS
MACHINES OF WAR
The number of monsters increases dramatically. Be prepared and be geared for it. This is where you'll want to use a lot of "AOE" attacks that damage lots of enemies at once.
* Guard of the Keep -- a guard decides to join you. He's almost impossible to keep alive.
* The Forward Barracks -- a side excursion with gold.
* Colonel Severyn -- help them fend off an attack
Treasure Pygmy/Treasure Bandit -always tries to escape, leaving a trail of gold to tempt you. If killed, drops magic items and gold.
Your follower's icon will glow and flash when they can add a new talent point.
Shrines are a "one use" item. Healing Wells will refill after a minute or two.
Monster talents change from level to level. In "Normal" mode, there are very few special attacks.
Snares - your character and followers are encased in a reddish cylinder and can't move until the spell/effect wears off
Elite (purple) mobs in Nightmare can come in groups of three.
Laser beams -- Some of the elites throw points of light on the ground that become rotating laser beams. Try to stay out of them.
Lava pools -- Some of the mobs create lava pools. Try to stay out of them
Green goo -- not good for your health. Stay out of that, too.
Stone walls -- restrict your movement
Frost traps - freeze you for a short period of time, during which you lose any generated talents (like rage for the Barbarian)
Each level drops its own unique things
No pages drop for Normal level
Nightmare level: pages drop