Poison 
Comes in many different potencies.  Many of the upper level creatures' poison isn't too deadly and will cause an infected character to lose a few hit points every few seconds.  This level of poisoning is usually easy to cure with magic.  However, one must beware of creatures who deliver deadly poisons, which can quickly kill a character and is almost impossible to cure without the aid of the Clerics in the city or an experienced Cleric character.  Poison can wear off (a high constitution helps this) but does not wear off very quickly. 

Disease  
As with Poison, Disease can be inflicted at different levels.  Disease usually takes fewer hits from an inflicted character, but is much harder to cure and remove.  It also takes longer to wear off. 

Stone 
Like Medusa in Greek mythology, some monsters can turn characters to stone.  Getting unstoned either requires a special spell or a trip to the morgue.  One point of Constitution will always be removed from the character being unstoned.  Getting stoned is not the same as Rocking a character (which requires some sort of teleport/movement spell). 

Age 
Will age the character, possibly many years. 

Paralysis 
Paralysis is very dangerous.  The player cannot fight or leave the room.  Additional defensive bonuses due to the players ability to move, parry, etc. are lost so the monster has a better chance of hitting you.  Paralysis will wear off after some time (a high constitution helps it to wear off more quickly) but quite often not before your character is dead. 

Electrocute 
Some monsters have the ability to Electrocute a character when they make physical contact.  This form of attack can be very dangerous and is usually delivered by water creatures. 

Drain 
This attack will attempt to reduce one or more of a character's stats, or even drain hit points! 

Breath 
Being fire, cold, electrical, or gaseous, this will cause extra damage to your character in addition to the monster's attack.  Monsters will use their breath attack more often if they feel threatened. 

Acid 
Some monsters have the ability to spit acid.  This form of attack is rare and requires good aim, but when successful, it can be very deadly. 

Spells 
Can be very dangerous, especially if the monster is of a high level.  Since monsters have no guild spell restrictions, certain monsters can cast a wide variety of spells. Beware that high level spell casters can even throw spells of the likes that even the guilds do not know! 

Critical Hit 
Will do extra damage to your character - quite often enough to kill you. 

Backstab 
Many thieving monsters have the ability to hide in the shadows and backstab your character for extra damage. 

Destroy Item 
Some monsters can destroy items you are carrying or wearing.  The chance of the item being destroyed is based on the power of the item and the power of the monster.  If you find a powerful item, a weak monster should not be able to destroy it.  Monsters that destroy items and inhabit the lower levels are greatly feared because they can destroy any item! 

Steal Item 
Thieving monsters are good at this.  They can only steal items that you are carrying, not items you are wearing.  If they get a clear shot at stealing an item, they will pick the easiest one to grab at that time.  If a monster grabs an item and feels it's worthless, it'll bash it against the wall, destroying it, and continue to fight with you, hoping to steal another item.  If the monster feels that the item is valuable, it will attempt to run away. 

Steal Gold 
Thieving monsters aren't always brave, so if they steal gold from a character, there's a good chance that they will run away.  That is if your character isn't fast enough to catch them! 

For both stealing attacks above, the chance is based on the monster's dexterity, thieving ability, and your dexterity and thieving ability.  So, a thief has less chance of being robbed than a non thieving character.  Gold or other Items held in the bank are safe, but unavailable while in the Dungeon. 

Behead 
Rumor has it that some creatures from the depths have the ability to remove an adventurer's head from their body with a precise attack.  Creatures that have this ability are very rare, but be warned that if you cross them, they will probably be very good at it! 

Note: Creatures that steal from you also have a habit of taking whatever gold they are protecting with them.  This can include a treasure chest as well!  However, an extremely dexterous character can quite often beat the monster to this, causing them to leave their booty behind!