Monster Power is a number that approximately reflects how strong and dangerous a monster is. An average monster has a Power of 1, strong monsters can have Power of 2 to 3, and weak monsters might have as little as 0.5, or very occasionally less. This value is then multiplied according to the monster's Rarity:

Normal: 1
Magic: 2
Rare: 5

Unique monsters always have 20 Power.

Slows are modifiers from Debuffs that cause actions to take longer. Slows can apply to a specific stat (such as attack speed or movement speed) — if a specific type of slow is not specified, it applies to everything the affected entity does. Slows are always multiplicative with each other.

Higher Rarity enemies are less affected by Slows:

15% less Slow effect on Magic monsters
30% less Slow effect on Rare monsters
50% less Slow effect on Unique monsters

Additionally, Slows have 10% less effect on all monsters for each player in the area beyond the first and monsters cannot be slowed below 25% of their base speed.

Flasks can be used to recover your Life or Mana. Flasks are not consumable, but require charges to use. Flask charges can be regained by killing enemies.

Normal enemies grant Flask charges equal to half their Power
Magic enemies grant Flask charges equal to their Power
Rare and Unique enemies grant Flask charges equal to twice their Power

Checkpoints and Wells completely refill Flasks when activated. Flasks can only hold charges while in a Flask slot.

Curses significantly Debuff affected targets. By default a target can have one Curse on them at a time.

Higher Rarity enemies are less affected by Curses:

15% less Curse effect on Magic monsters
30% less Curse effect on Rare monsters
50% less Curse effect on Unique monsters

Exposure is a type of Debuff that lowers the affected enemy's Total Elemental Resistances. By default it lowers Resistances by -20% and lasts for 4 seconds, though some sources of exposure can override these values. Like most sources of lowering Resistances, this can cause the enemy's Resistance to become negative.

Higher Rarity enemies are less affected by Exposure:

15% less Exposure effect on Magic monsters
30% less Exposure effect on Rare monsters
50% less Exposure effect on Unique monsters

Corpses are left behind by slain Enemies and can be used or consumed by a number of different Skills or effects, though Corpses of Unique monsters cannot be destroyed or consumed. Generally, slain Allies do not leave usable Corpses.

Valour is used to fuel Banner Skills. Killing an enemy with an Attack generates 1 Valour, and Banners passively gain 1 Valour per second while a Unique enemy is in your Presence. You can only gain Valour once every 0.5 seconds, and a Banner skill cannot gain Valour while its Banner is placed.

Each Banner has 50 maximum Valour by default. If you have multiple Banner skills active, each gains Valour separately.

Maps containing a Delirium Mirror can have the mirror walked through to unleash the rolling Delirium fog ring across the current area.
You must stay within this fog as it expands to maintain the Delirium or it will disappear.

Magic monsters are able to gain Delirium specific Modifiers while inside the fog.

Rare or Unique monsters in the fog may manifest Delirium Demons. Manifested Delirium Demons inhabit the monster's body to occasionally use Skills of their own.

Fractured Mirrors may be found within Delirium Fog, summoning extra monsters while Fracturing Mirror Shards are found at set depths, with more varied rewards.

Defeating Map Bosses has a chance to summon a Grand Mirror on a nearby map.

Contains a Unique Map layout which may contain specialised rewards.

Unique Maps cannot gain additional content, Essences, Shrines, or Strongboxes.

Contains a Hideout that can be fully cleared to unlock the Hideout for personal use.

All monsters in the area are at least Magic.

Rogue Exiles are dangerous foes that wander Wraeclast and Maps.
They have access to the same Skills, Items, and Uniques that you do. This can make them very formidable opponents - however, if they can be defeated they will drop a full set of gear, including any Unique equipment that they were using in combat.
If any Rogue Exile manages to defeat you, they will portal away, taking their equipment with them.

Azmeri Spirits are the spirits of various Azmeri Animal Guardians.
Azmeri Spirits appear as wisps that flee as you approach them. Normal or Magic Monsters the Spirit comes into contact with become Spirit-Influenced, making them more powerful.
Sometimes the Spirit will reach and Possess a Rare or Unique monster, making them substantially stronger and rewarding while occasionally summoning spiritual animals depending on the type of Animal Spirit they represent.
Possessed monsters are more rewarding depending on how many Spirit-Influenced monsters were defeated leading up to possession.

A Possessed monster is a Rare or Unique monster that has had an Azmeri Spirit enter and empower them. The possessed monster is empowered by the spirit and gains bonuses depending on the type of Spirit that possessed them. Occasionally the possessed monster will summon spiritual animals depending on the type of Animal Spirit they represent.
Possessed monsters are more rewarding depending on how many Spirit-Influenced monsters were defeated leading up to possession.

A Spirit-Influenced monster is a Normal or Magic monster that has been passed through by an Azmeri Spirit. These monsters are empowered by the spirit and gain bonuses depending on the type of Spirit that passed through them.
Defeating Spirit-Influenced monsters makes the resulting Possessed monster more rewarding.

The final Rare Monster slain in a Map Area has a chance to drop a Waystone equal to the tier of the Waystone used to create that Map Area.

The chance diminishes the tier of the Waystone used to the create the Map Area, but can be increased again by adding modifiers to Waystones, additional modifiers on Waystones can give increased chance for Waystones to drop in the Map Areas they create.

Delirious players are assaulted by illusions, making combat more difficult. Higher delirium causes monsters to deal more damage and have additional Toughness. It can also cause additional monsters to appear or can grant additional modifiers to existing monsters. Monster item drops are improved by higher delirium.

Maps within Fog Banks will Deliriousness as Map Bosses, Rare Monsters or Unique Monsters are killed.

Players with Mounting Greed gain increased Rarity of Items on kill up to a limit of 100%. This increased Rarity decays over time.

Higher Rarity monsters grant a greater amount of Rarity.

Monster Rarity increases the chance for monsters to be Rare and Magic.

Monster Rarity also increases Monster Modifier Chance for Rare Monsters.

Monster Minions are any monsters that are part of a Rare Monster Pack.

These monsters receive one modifier from each Rare Monster in the Pack.

Monsters summoned or created by other Monsters do not count as Monster Minions.

Rogue Exile Hunting Grounds contain 2 additional Rogue Exiles and 5 additional Packs of Rare Beasts.

Additional Rare Monsters can spawn in Rare Monster Packs with larger pack sizes.

Each Rare Monster in the Pack provides one modifier to the Pack's Minions.

Additional Rare Monsters in Essence Packs also carry Essences.

Exceptional Items have Quality over maximum or an additional Augment Socket.

High Tier Rare Items found have a chance to instead drop as an Exceptional Normal Item.

Secured Strongboxes are guarded by additional Packs of Rare Monsters and may roll Modifiers which add Monster Modifiers to any Rares guarding the Strongbox.