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.
- 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.
- 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
- 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)
- 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.
- 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.
- 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.
- Energize -- Restores pc's mana.
- Heal -- Restores pc's (and maybe other monster's) hp.
- 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.
- 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.
- 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 :)
- Suicide -- Kills the monster.
- Hunt -- The monster can attack another monster of specific kind and heal by killing it.
- 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.