I needn't carry a stone around the world to understand its hardness. I need only know it is a stone. 
Demon Knights #20 is an issue of the series Demon Knights (Volume 1) with a cover date of July, 2013. It was published on May 8, 2013.
Synopsis for "The Stone Door"
In Themyscira, Shining Knight suffers from nightmares of her own vampirism while Exoristos is invited to return to her homeland. In the city's library, the Demon Knights determine the Holy Grail's possible whereabouts - an uncharted island in the middle of the Mediterranean Sea. They set sail and, after two days, find a fleet of sunken ships. Xanadu discovers and dispels an invisibility enchantment and a hidden island appears.
On shore they are almost immediately attacked by a giant octopus named Architeuthis. Once the monster is destroyed the group finds a stone door adorned with glyphs. They determine that the glyphs are of Merlin's design and one in particular represents the Holy Grail.
Appearing in "The Stone Door"
Featured Characters:
- Demon Knights (Also in a vision)
- Etrigan (Jason Blood)
- Exoristos
- Horsewoman
- Madame Xanadu
- Shining Knight (Ystin) (Vampirized) (Also in a dream sequence)
- Vandal Savage (Rejoins team)
Supporting Characters:
- Amazons of Themyscira
- Queen Hippolyta
- Khronika (Single appearance)
- Brickwedge the Horse
- Vanguard the Flying Horse
Antagonists:
- Architeuthis (Real name revealed) (Dies)
- Cain (In dream sequence only)
Other Characters:
- Atla (Flashback only) (Cameo)
- The Merchantman (Single appearance) (Unnamed) (Dies in flashback)
- Vampires (In dream sequence only)
- Hydras (Mentioned only)
Locations:
- Prime Earth
- 1000
- Mediterranean Sea (Flashback only)
- Crete (Mentioned only)
- Mediterranean Sea (Flashback only)
- 1043
- Mediterranean Sea
- Themyscira
- Library of Themyscira
- The Royal Palace of Themyscira
- Themyscira
- Uncharted Island (First appearance; unnamed)
- Britain
- Camelot (Destroyed) (Mentioned only)
- Mediterranean Sea
- 1000
- Sphere of the Gods (Unnamed)
- Dream Realm (Behind the scenes)
Items:
- Bracelets of Submission
- Caliburn
- Exoristos' Hammer
- Hippolyta's Tiara
- Holy Grail (Mentioned only)
- Library books
- "Kamasutra" (First appearance)
Vehicles:
- Knights' Amazon ship (First appearance)
- Merchant ship (Flashback only)
Events:
- The Fall of Camelot (Mentioned only)
Concepts:
- Magic
- Middle Ages
- The Red (Behind the scenes)
Trivia
- Given access to Themyscira's vast historical library, Vandal Savage went directly to the how-to section to find and read Kama Sutra.