Note that there are more titles on sale than the 3 pages worth that I have linked to. Only some of the games are horror despite the Halloween theme. Some titles of interest:
Darksiders Complete (1 + 2 + DLCs) (85% off) $9 http://store.steampowered.com/sub/18777?snr=1_8_4__501
State of Decay & DLCs (67% off) $6.79 for the base game http://store.steampowered.com/app/241540/
Alice Psychosis Returns (75% off) $5 http://store.steampowered.com/app/19680/
(I cannot recall seeing this on sale before)
Amnesia The Dark Descent (75% off) $5 http://store.steampowered.com/app/57300/?snr=1_7_7_230_150_1
STALKER Bundle (50% off) $20 http://store.steampowered.com/app/41700/?snr=1_7_7_230_150_1
Outlast (66% off) $6.80 http://store.steampowered.com/app/238320/?snr=1_7_7_230_150_...
Insurgency (50% off) $7.50 http://store.steampowered.com/search/?snr=1_5_9__12&term=pen...
(do terrorists count as monsters?)
Penumbra Collectors Pack (2 games) (80% off) http://store.steampowered.com/search/?snr=1_5_9__12&term=pen...
Gemini Wars (75% off) $3.75 http://store.steampowered.com/app/216130?snr=1_8_4__501
Fallout 3 (75% off) $2.50 http://store.steampowered.com/app/22300/
(The GOTY version is much dearer)
Ghostbusters (70% off) $3 http://store.steampowered.com/app/9870?snr=1_8_4__501
Space Hulk Ultimate Pack (70% off) $12 http://store.steampowered.com/sub/45516/?snr=1_5_9__403
Skyward Collapse Comple (80% off) $1.40 http://store.steampowered.com/sub/47483/
Shattered Haven (80% off) $2 http://store.steampowered.com/app/234370/?snr=1_7_7_151_150_...
Battle Vs Chess (85% off) $3 http://store.steampowered.com/app/211050?snr=1_8_4__501
(this might have extra DRM, since its a Topware title)
Dark Fall 1&2 (80% off) $1.60 http://store.steampowered.com/sub/35378/
Rather than create another thread, I noticed there were a modest number of discounted games (mostly Kalypso titles) on sale at the Bundlestars store. http://www.bundlestars.com/store/
These include Dark for $4 (90% off), Blood Knights $2.25 (85% off), hegemony Rome $6 (80% off), and Demonomicon The dark Eye for $6 (85% off) http://www.bundlestars.com/store/demonicon-the-dark-eye/
GreenGuy is always worth checking out. Here is a 22% off code: SLICKD-EALS22-OFFGMG
Wolvenstein (50% off) $30 http://www.greenmangaming.com/s/au/en/pc/games/shooter/wolfe...
75% off Europa Universalis http://www.greenmangaming.com/europa-universalis-iv-deals/
75% off other Paradox titles http://www.greenmangaming.com/paradox-deals/
66% off War of the Vikings $8.50 http://www.greenmangaming.com/s/au/en/pc/games/action/war-vi...
EDIT: Adding more Steam titles that i have found or others suggested.
The Witcher 2: Assassins of Kings Enhanced Edition $3.99 http://store.steampowered.com/app/20920/
The Witcher: Enhanced Edition Director's Cut $1.99 http://store.steampowered.com/app/20900/
Infestation: Survivor Stories $3.74 (75% off) http://store.steampowered.com/app/226700/?snr=1_7_7_151_150_...
Legacy of Kain Collexion (4 games for $4, 80% off) http://store.steampowered.com/sub/49915/
natural Selection 2 (75% off) $6.24 http://store.steampowered.com/app/4920/?snr=1_7_7_151_150_1
Spore (75% off) $5 http://store.steampowered.com/app/17390/
Super Haus of dead Ninjas (80% off) $1.49 http://store.steampowered.com/app/224820/?snr=1_7_7_151_150_...
Dead Space 2 (75% off) $5 http://store.steampowered.com/app/47780/
Cthulhu Saves the World & Breath of Death VII (67% off) $1 http://store.steampowered.com/sub/8848?snr=1_8_4__501
Elminage Gothic (60% off) 3.99 USD http://store.steampowered.com/app/291960/
(Japanese Wizardry clone, reasonably hard)