Monster attacks

The content of your game is very important once you overcome the minor technical problems of writing the engine. ;) I'd say that a lerge number of interesting and distinct monster kinds makes for a very large part of a good roguelike game. And one of the most obvious things that differs the monsters are their special attacks and their effects.

The list is not supposed to be a list of all attack effects possible in a roguelike, nor a list of attack effects that every roguelike should have. It's supposed to serve as an inspiration for your own ideas.

Damage

Hit -- Just your standard 'substract an amount of hp' attack.
Wound -- Create a wound with long-term effect, ie. bleeding.
Electric shock -- The attack ignores any metal armor.
Torch -- The pc is set on fire.
Fire attack -- Can damage scrolls/books/potions.
Shoot -- Standard ranged attack.
Poison -- A stat of the pc decrease for some time.
Breathe fire -- damages monsters, say on 3 squares in a given direction.
Disease -- Prevents health regeneration, possibly other effects too. Spreads by proximity.
Basilisk stare -- Turns an item/pc into rock.
Earthquake -- Damages all creatures around.
Demi, Quart -- pc loses half or one-fourth of his current hp
Fart bomb -- damages all creatures nearby every turn, for several turns.

Stats/Numbers

Drain stat -- Lowers one of pc's statistics.
Permanent drain stat -- Lowers one of pc's statistics permanently.
Drain level -- Lowers pc's experience.
Nexus -- Swaps around PCs stats
Swizzle -- Swap the PC's HP and mana totals
Make hungry -- Makes the pc hungrier.
Drain magic -- Decreases magical pluses, turns scrolls into empty scrolls, potions into potions of water.
Drain power -- Decreases pc's mana.
Encumber -- make Pc's carrying capacity smaller

Mental/Inaction

Paralyze -- The pc can't do anything for several turns.
Sleep -- Like Paralyze, but it ends when pc is attacked.
Fear -- The pc becomes afraid.
Confuse -- The pc moves around randomly.
Madness -- Scares the target and drives it insane.
Seduce -- Makes the pc take off his armor.
Charm -- Makes the pc unable to attack the monster.
Silence -- The pc can't cast any spells for a number of turns.
Blind -- The pc becomes blind for some turns.
Amnesia -- Causes amnesia.
Aggravate -- Makes all the monsters around come here.
Stun -- Makes the pc unable to fight for the next turn.
Tumble down -- Makes the pc unable to walk for the next turn.
Slow -- the player isn't paralized but acts slower
Grab -- The pc can't move away from the monster.
Posess -- Allow the monster to possess the player for N turns. During this time the PC would be computer controlled and attack the enemies of the possessing monster.
Disorient -- Paint the screen upside down (this one could be great)

Inventory

Steal -- The monsters takes an item from pc's inventory.
Pickpocket -- The monsters takes some gold from pc's inventory.
Steal spell -- Makes the pc forget one spell and gives it to the monster.
Sticky curse -- One piece of pc's equipment becomes sticky-cursed.
Krezus curse -- Turns an item into gold.
Disarm -- Makes pc drop his weapon.
Scatter equipment -- Makes PC's equipment (some if not all) to be thrown around.
Give item -- Makes PC to accept an item (e.g. loadstone, timed explosive or something else not so useful).
Corrode -- Attack can damage metal armor.
Acid attack -- Attack can damage any armor.

Terrain

Trap -- Creates a trap.
Darkness -- Makes the squares around pc dark.
Light -- Makes the squares around pc lit.
Alter ground -- turns grass to rock, or poisoned river to clean river, or whatever.

Monster

Clone -- Creates a copy of the monster.
Mimic -- Makes the monster look/behave/have stats like the pc.
Mutate -- The pc grows an additional organ that changes his abilities
Polymorph -- Targeted at other mosnters, changes their kind.
Turn -- Turn the pc into a chicken (or something else) -- he keeps his HP, but can't attack.
Rise dead -- Creates zombies from monster corpses.
Summon -- Summons monsters.
Divide -- split into two (distinct from summon/clone in that each half has half the power of the original)
Merge -- take two monsters and make a stronger one from them
Cooperate -- an attack that needs several monster to act at the same time
Throw buddy -- monster takes a smaller buddy and throws it at target. Both projectile and target takes damage
Animate -- raise weapons, armour, rocks, etc. to fight the target. Possibly their weapon, possibly whatever was lying around.

Benificial(?)

Energize -- Restores pc's mana.
Heal -- Restores pc's (and maybe other monster's) hp.

Moving

Push back -- The pc is pushed back by the force of attack.
Teleport -- The pc is teleported into random place.
Teleport from -- Monster teleports away from the pc.
Teleport to -- Monster teleports to a sqare next to the pc.
Teleport to self -- The PC is teleported to a square beside the monster.
Switch places -- Switches places of pc and monster.
Teleport level -- pc is teleported to another level
Wind -- creates some kind of tornado that takes monsters and objects with it in some direction
Fly/levitate -- avoid terrain, or get out of reach of PC
Whirlwind -- pick up items from the floor and move them around randomly, possibly hitting creatures in the way
Phase out -- target disapears completly for a short time. No other adverse effects.

Temporal

Time warp -- time clocks back at the point where the player entered the level
Balefire -- Kills target and undoes the target's last N moves.

Sacred

Soul Steal -- Steal the PC's soul, making the gods ignore his praying.
Sacrifice -- If PC stands on an altar, sacrifice. Like in adom.
good music -- increases the stats of all nearby friendly monsters
bad music -- damages and stuns the target. See sound attack :)

Additional Effects

Suicide -- Kills the monster.
Hunt -- The monster can attack another monster of specific kind and heal by killing it.

Other

Idle -- No attack at all.
BSoD -- Crash the computer the game is running on (portability issues here)
Dispel -- Removes any magical effects from pc.
Armour-penetrating -- Like a normal attack, but bypasses (non-magical?) armour.
Curse -- inflicts the player with various curses that have negative effects but only lasts a short time
Hide -- become difficult to see unless really close (chameleons?)
Engluf -- pc is englufed or swallowed, so it can only fight one creature and cannot move. May be combined with Condemn to simulate digestion.
Condemn -- Put's a timer on the PC. When it expires the PC dies. This timer could either be dispelled or canceled when the monster dies.