Because the Comics Code had gotten revised and now allowed most horror conventions to return, with a few restrictions, DC had a renaissance of the horror genre that began in 1968 and peaked in the mid-70s.
Fortunately, at the time, popular horror in other media was more psychological or supernatural, which could be done in comics with no fuss.
However, with the rise of gore in slasher films and such, the comics couldn't keep up and eventually began to look tame in comparison.
By the early '80s, they were gone. But not without leaving a legacy.
Swamp Thing, Phantom Stranger, Madame Xanadu and many Sandman supporting characters featured prominently or first appeared in the pages of DC's horror line.
Indeed, virtually the entire early Vertigo line was somewhat descended from these books, as Karen Berger (the Godmother of Vertigo) got her start at DC as an editor on House of Mystery in 1981.

The core titles were:

House of Mystery (rebranded as a horror anthology with #174) - May/June 1968-Oct 1983
The Unexpected (Mad Mod Witch begins with #108) - Aug/Sept 1968-May 1982
The Witching Hour - Feb/Mar 1969-Oct 1978
The Phantom Stranger - May/June 1969-Feb/Mar 1976
House of Secrets (relaunched as a horror anthology with #81) - Aug/Sept 1969-Oct/Nov 1978
Ghosts - Sept/Oct 1971-May 1982
-For a time from the end of 1970 until late 1972, these titles also carried a bat-symbol on their covers to designate they were part of DC's horror line.

Swamp Thing - Oct/Nov 1972-Aug/Sept 1976 (to be revived in 1982 at the end of the 70s horror wave and find even better success under Alan Moore, leading to the 'British Invasion' and '90s Vertigo horror resurgence).
Secrets of Haunted House - Apr/May 1975-Mar 1982 (with a year break in 1976)
Doorway to Nightmare (arrived in 1978 and was cut short by the DC Implosion (which also took Witching Hour and House of Secrets) - Jan/Feb 1978-Sept/Oct 1978

Fringe titles included:

Weird War Tales
Weird Western Tales
Weird Mystery Tales
-these weren't under the horror umbrella but rather a mini-line unto themselves.

Dark Mansion of Forbidden Love (later renamed Forbidden Tales of Dark Mansion)
Sinister House of Secret Love (later renamed Secrets of Sinister House)
-likewise, they were a sub-imprint that mixed the horror stuff with DC's romance stuff at the time. The first few issues of each (before the title changes) also had very striking gothic novel style covers. Similarly, while the horror titles briefly carried the bat symbol on their covers, the romance comics carried a heart symbol and these two had a silhouette of a house or manor (a staple of gothic fiction).

And other titles include:
Tales of Ghost Castle (a three issue series from 1975)

Books that carried the 'DC Witching Hour' logo:
Witching Hour from Oct/Nov 1970 to June/July 1972.
House of Secrets from Oct/Nov 1970 to Oct 1972.
House of Mystery from Nov/Dec 1970 to Oct 1972.
Phantom Stranger from Nov/Dec 1970 to Sept/Oct 1972.
Unexplained from Feb/Mar 1971 to June 1972.
Ghosts from Sept/Oct 1971 (its first issue) to May/June 1972.

Further reading:
DC Comics' Horror Titles of the Seventies
Follow Me into Weird Worlds: DC’s Horror and Supernatural Comics

Honorary Mentions:
Elvira's House of Mystery (1986) #1-11, Special #1
Night Force (1982) #1-14
Swamp Thing (1982) #20-64

There are trades available for the following:

Ghosts
Showcase Presents Ghosts #1 -reprints #1-18

House of Mystery
House of Mystery - DC Treasury Edition (1973) -various reprints
House of Mystery - Bronze Age Omnibus #1, 2 -reprints #174-226
Showcase Presents House of Mystery #1-3 -reprints #174-226

House of Secrets
House of Secrets - The Bronze Age Omnibus #1, 2 -reprints #81-154
Showcase Presents House of Secrets #1, 2 -reprints #81-119

Night Force
DC Comics Presents Night Force #1 - reprints #1-4
Night Force: The Complete Series -reprints #1-14

Phantom Stranger
Showcase Presents Phantom Stranger #1, 2 -reprints #1-41, and a few appearances

Secrets of Sinister House
Showcase Presents Secrets of Sinister House #1 -reprints #1-18

Swamp Thing
Roots of Swamp Thing -reprints Swamp Thing (1972) #1-13
Secret of the Swamp Thing -reprints Swamp Thing (1972) #1-10
Swamp Thing: Dark Genesis -reprints Swamp Thing (1972) #1-10
Swamp Thing The Bronze Age #1, 2 -reprints Swamp Thing (1972) #1-24, and a few appearances
Saga of the Swamp Thing #1-6 -reprints Swamp Thing (1982) #20-64

Weird War Tales
Creature Commandos: A Weird War Tales Collection -reprints Creature Commando stories
Showcase Presents Weird War Tales #1 -reprints #1-21

The Witching Hour
Showcase Presents The Witching Hour #1 -reprints #1-21

Previously... in the last appreciation thread...