Dark Beast
Dark Beasts are easy to kill by the time you have 90 slayer, they are located in the dungeon below the mourners house in West Ardougne, you need to at least start Mournings End Part II and wear the full mourner clothing set, you can not wear anything else to enter the mourners house with the exception of a ring.
It is by far easiest to wear proselyte and use protect melee prayer a God Sword is the weapon of choice to kill Dark Beasts but a whip will work fairly well, you can use Guthans but you will take a lot of damage and be using the spear a lot and therefore slow training down considerably, I always wear a ring of wealth fighting Dark Beasts because they drop dark bows.

Dark beasts look like a cross between a gorilla and a rhinoceros, they are slow clumsy animals that attack using Mage and melee attack styles.

Fighting techniques
Use melee protect prayer and pot up before entering the area, as soon as you enter all beasts will attempt to attack you, if you attack 1 first it will mage you and normally cause damage (about 8) then continue to melee attack, if you just run in they will automatically attack melee and this can save you the mage damage, they will all stare at you as you slaughter their friends but it is non multi area so they can not help, as soon as you have killed 1 another will start to attack, again if you let them attack it will save you the mage damage, you may need to move to give them space to attack you as I said they are slow and clumsy so need a bit of space.
They never get used to you so will always be aggressive, you can just keep topping up your prayer and watch the fight, they rarely have good drops but drop more blue charms than any other colour, I had 19 blue charm's from 170 kills so not a great dropper but proportionally good compared to other charms.

Summary of: Dark Beast
Temperament: Constantly Aggressive
Combat Level: 182
Attack Style: Melee
Use Guthans: Possible but not recommended
Recommended armour: Proselyte or initiate
Slayer Level to kill: 90
Slayer XP: 225
Max Hit: N/K
Examine info: From a darker dimension
Drops: Adamant javelin, Adamant square shield, Adamantite bar, Air rune, Attack potion, Black 2h sword, Black axe, Black battleaxe, Blood rune, Blue charm, Chaos rune, Coins, Crimson charm, Dark bow, Death rune, Death talisman, Dragon medium helm, Dragonstone, Gold charm, Green charm, Grimy cadantine, Grimy dwarf weed, Grimy guam, Grimy harralander, Grimy irit, Grimy kwuarm, Grimy marrentill, Grimy ranarr, Grimy tarromin, Law rune, Loop half of a key, Nature rune, Nature talisman, Rune 2h sword, Rune Arrows, Rune chainbody, Rune full helm, Rune sq shield, Runite bar, Shark, Shield left half, Silver ore, Steel Arrow, Tooth half of a key, Uncut diamond, Uncut emerald, Uncut ruby, Uncut sapphire
Runescape monster database, your help needed.
You may have noticed my monster database, you may have noticed there are only half a dozen monsters in it, the problem is there are so many monsters and so few hours in the day.
I would like to make an exhaustive database of monster information, this is something all the fan sites lack in 1 area or another, I want to bridge this gap but I need your help.
If you have any information about monsters I would love to hear from you, I want to know everything from details how to get there, what their max hit is, special weakness, drops, requirements to kill and killing techniques.
I will start posts with just the monster name and you can add comments, I will review and add the comment, I will then add them to the initial post when I have sufficient time and information.
If there is a monster that I have not yet added a heading for, put the monster name and details as an attachment to this post and I will add a heading for that monster.
Remember I want all information no matter how small so we have the one stop shop for all monster details.
Detailed description how to get to Abyssal Demons.
This is a question many people ask, it is fairly easy to get there but not without its dangers, this was originally added by myself as a comment to another post but due to its popularity I have added it as a post.
There are 2 places the Abyssal Demons live, the first is the slayer tower in Canafis.
Enter Canafis area via the temple on the Salve (East of Varrock), this is the temple you killed the big dog for the Priest in Peril quest, if you haven't killed the dog you cant get to Canafis or the Slayer Tower.
Assuming you have done the quest when you leave the lower level of the temple take the path north, this will take you to the slayer tower, the first monster you will come across are the crawling hands (there are only 2 aggressive monsters in the slayer tower and I will make you aware of these as you go along).
There is an agility shortcut in the second room if you have the required level you can climb up there, (Beware that brings you out with the aberrant specters these are aggressive and dangerous), if you haven't got the required agility level continue to walk around the rooms past the Banshee's and up the stairs.
The next floor you walk around and when you enter the room with the Infernal Mages there is another shortcut, if you can use this one do so as the other options of using the stairs are dangerous.
If you don't have the agility level walk around the rooms and past the Bloodvelds, in the big Bloodveld room there is a door, this door is to the Aberrant Specters, Be carefully these are very dangerous, if you can not wear a nosepeg you stand a very good chance of dying, a tip is to use a busy world so all the specters are being attacked if they hit you they use mage so they can get you from a distance and can attack you for a long way.
Just inside the door is the shortcut from the crawling hands, if you use the shortcut it is the same as using the door you need to run west then south and enter the stairs as quickly as possible, taking a hit from the specters without a nose peg will effect your stats severely, if you use the agility shortcut to get up and you take a hit you will need to take the stairs back down.
The spectres are the worst once you have got past them and up to the last floor the worst is over, the next room has Gargoyles in these are aggressive and will hit you melee style so wear protect from meele if you have it or just run like hell to the door at the north side enter the door and you are now safe.
The other side of the door is the Nechryael and the top of the agility shortcut from the infernal mages, just walk around the rooms and you will be with the Abyssal Demons.
Watch the players fighting and tell them how great they are if you wish but don't ask for free whips, it has taken a lot of work for them to get that level and noobs asking for a free whip is a pain after a while.
This is however a good way for the lower levels to get some extra cash, a lot of things you may value are left by the higher level players feel free to pick them up and sell if you wish.