I collected some info's about this area.

Start: 08. October
End: 14. November

Halloween boss names are often "daimonic", you find the real names, if you read them from behind.
Maps have also special themed random drops.

Garrrgghville
Has a small shop with a blacksmith and one quest npc.
Mayor Clobber - Holiday Spirit Quest - Reward: Halloween Ring? (forgot if i got it from here)

Okonkwo maze (south of Garrgghville) level 42-51
Walking over stones teleports back to entrance. There are also teleporter leading deeper in there. Easiest to describe is go southwest corner, southeast corner, try to go east, west there ...
demonic pumkins, headless horsemen (super fast, around corners, often several very close together)
Boss: Annahdd (level 51) - can drop Halloween pants

Okonkwo cave (sever hole in Garrrggville) level 40-50
spiked pit traps, giant spiders
2 stairways down to Okonkwo Underground
one leads to small separated room, other to boss area
Okonkwo Underground
spiked pit traps, giant spiders
Boss: Levaburz level 50 - can drop Halloween pants

Deathly Hollow city area (north of Garrgghville) Level 72-75
inn bar has an opening you can go through

Deathly Hollow graveyard area level 72-82
Lost Souls, Skeleton fighters
Boss: Rignol level 73, can drop pants?

Cursed Reaches / Gloomy Path level 1-8
lots of different low level mobs
way to enternal Graveyard leads from Deathly Hollow graveyard besides the swamp to eternal graveyard

Eternal Graveyard level 6-7
This area has four graves leading to different caves in Cavern of Despair. There is sometimes one named skeleton mage called Walace, who drops a halloween chain+2 on first kill.
Skeleton Fighters, Skeleton Mages
Boss: Walace drops halloween chain+2 (1-drop)

Cavern of Despair (highest instance level 82-110 mobs)
This area has four entrances, four open graves from eternal graveyards leads to four different crypt areas. There are four different instances, players can go in depending on their main level.
Highest instance mobs from level 80 to 110
Slimes, Brown Bats, Spiders, Mud Hands, Vampires

South crypt is the left lower left entrance. There is a secret passage to southwest and later there is a gate you can open with the south crypt key. This is mostly unimportant to go here, except to get the first key.

East crypt is the lower right entrance. You need the east crypt key here to advance deeper. Kill monsters in east, south crypt, … to find it. With key you can pass through the gate here, but better go first in west crypt and get the north crypt key there. If you have both keys, go the room behind the east crypt gate. Here are several secret walls. Three secret passages, two to the north are dead ends, the one to the south leads to the north crypt gate. Kill monsters here for the west crypt key. Here is also a not working teleporter (bug?).
      
North crypt is upper right entrance. This way is blocked by rocks. You need east crypt key and north crypt key to reach the area behind the blocking rocks and you can only reach this area behind via a passage from east crypt. In there you need to kill monsters to find west crypt key. Here are also 2 teleporters to the final room, but they only work when you have south crypt key. Teleporters are next to the 2 pillars in room.

West crypt is upper left area. There is a blocking gate, but searching north you find some secret walls. Traveling there you can reach the area behind the gate. Here is also a teleporter. Camp monsters here to get the north crypt key. Brown bat drops it for sure. Later with the west crypt key you can use the teleporter here.

There are 2 teleporter rooms looking same. But dropping a gold coin or use /mapinfo you see they are different. Go 2 times the teleporter to the right. All other teleporters leads back to different places in crypt.

The final room is called Hidden Chamber
Slime, Brown Bat, Mud Hand, Vampire Chamber Guard
Boss: Vampire Chamber Lord 110, boosted hp, can drop hallo gaunts

East key: First you need to kill monsters down there to find the east crypt key.
North key: Enter west(upper left) crypt, pass secret wall north. Kill monsters in room with 3 vampires and teleporter to get the north crypt key.
West Key: With east and north key go via east crypt passage to north crypt and farm west key.
South Key: With west key go back to west crypt and use the teleporter there, use two times the right teleporter. Farm here for south crypt key.
With all 4 keys go back to the north crypt via east crypt passage. Here are the final teleporters to the hidden chamber.

Here is also an older spoiler with screenshots from modularfish.

https://www.daimonin.org/4321/helpadvice-getting-crypt-keys-spoilers/

Graveyard level 9-14
Lost Souls, Vampirs
Boss: Trudi level 14

Madoom (use Haunted Plains teleporter) level 10-17
Headless Horsemen, Zombies

The Caves of Ouija level 24-27
guarded by Demelichs, some pits, sewer hole leads to Arachnoria

Arachnoria level 25-30
Giant Black Widows
Boss: Neothcim level 30

Mabon (ladder in Adeff Hills southwest of Haunted Castle) level 50-67
Giant spider (60-65)
Skeleton (61-67)

south area leads to
Boss: Nosrep level 66
north area leads to room with skeletons
east area leads to spiders and skeletons

Samhain (enter by Forgotten planes teleporter) level 90-95
Zombies and Lost Souls

Forest of Fane (west of Haunted Palace) level 96-103
Vampires
Boss: Ykcams level 103 drops halloween pants

Alban Elfed (enter from Forest of Fane) level 50-58
Evil Pumpkins
Boss: Noduwoh level 58 drops halloween pants

Omensdoom Sewers (sever hole in Adeff Hills west of Haunted Palace) level 95-110
Lost Souls, Zombies, Skeletons
Boss: Sisircts level 102
Zombies can drop nunchucks (one drop but must be looted from corpse)
frienly npc perma killed by mobs: Bob level 115

Hallow Tide (level 83-89)
Skeleton Fighters, Skeleton Mages
Boss: Rebbolc level 90, drops hallo pants?
?skels here can drop halloween shield, sycthe

Haunted Palace
different instances for 1-15, 16-25, 50-75, 76-95, 96-110
highest instance level 97-115 mobs, good exp
skeleton warriors, ghuls, zombies, enchanted guard

palace entrance key (kill skeletons in hallway till it drops)
Torture Room Door Key (from enchanted guard with reflect spells)
Amulet of Anon: anon poison vial quest, fill vial of poison, giv this anon

Cat’s Paw key
Quest cat’s paw’s journal
speak to Cat’s Paw, before you need jump the lava pit from stone to stone
Get journal from corpse (chained skeleton) by a large tree near the Screaming Moors.
Get Cat’s Paw key from cats's paw

first floor mansion key
Open door to small labyrinth (you need cats's paw key), find the chest

Palace master key: kill lost soul level 115

Narrow passage, behind secret wall library shelf to dank basement lvl 94-122
lots of different mobs
a hole under webs leads to Inferno Map
!warning Boss on highest level has extreme boosted hp
!fight could take up 45 minutes and you need 1k+ food
!for solo fighting boss better let level drain yourself and fight boss on a lower instance
!don't use teleporter over lava, if you don't wand to fight boss
Inferno Shadow Fire Demon ! very good exp

boss: Chaotic Tarrasque drops celvenate crystal
special attacks and boosted hp
celvenate crystal is needed to teleport back (apply obelisk), but vanish after teleport.
if you go back same place, you need to kill Chaotic Tarrasque again

Haunted Castle (far west of entrance, pass moore)
west of Forest of Fane, northwest of the Forest of Fane vampires, southwest of Hallowtide
Haunted Castle has 6 different instances (highest instance mob level 102-117)
Butler Smythe level 84
go stairs up and down
zombies level 102-108
Haunted Castle Basement
Wraith, Bone Devil, Skeleton Fighter, Skeleton Mage
Boss: Sharaks drops Amulet of Skulls

Headless Horseman drops a Horseman’s Head
!warning touching altar -9xx hp, mana, grace
walk over the chequered tiles with head, then drop head and continue walking
switch behind forth upper shelf next to gate
walking riddle from entry (pos 9,16) go numpad
9,9,9,3,3,2,2,6,6,2,1,1,1,2,6,6

Lost Soul level 108-113
drop candy in chest, apply chest again, after this you can pick up candy

Buttler Smythe transforms to Green Demon level 127 who drops Master Bedroom Key
go master bedroom, go stairs up

Apply locker to spawn a Skeleton level 101 to drop the Castle Key

Friendly Pumpkin has riddle
„what occurs once every halloween, twice every harvest festival and thrice every all Saint’s day“
5 poison pools are deadly, apply pool of poison down of Pumkin (17,13) -> take candle of light
with candle of light you can pass without burning

go stairs up, you got instantly attacked by multiply spiders
Giant Spider level 93-106
Giant Black Widow level 113 -> drops silk spider's web on first kill.

Talk to Smythe for Haunted Castle Reward, choose between 5 robes.
Warrior, Mage, Priest, Archer and neutral robe
Talk to Magic Mirror to leave