Question: Thunderspire Labyrinth is the second in the three part series of adventures introducing the new 4th edition "Dungeons & Dragons" concept of what, he adventure, was written by Michael Mearls, a writer and designer of fantasy role-playing games (RPGs) and related fiction?

Context articles: Dungeons &amp; Dragons (IDW Publishing)
List of Dungeons &amp; Dragons modules
Dungeons &amp; Dragons Adventure Game
Thunderspire Labyrinth
Pyramid of Shadows
Keep on the Shadowfell
Dwellers of the Forbidden City
Dungeon Crawl Classics
Orcus (Dungeons &amp; Dragons)
Mike Mearls

Dungeons & Dragons is a comic book series by IDW Publishing, based on the 4th Edition core setting of the "Dungeons & Dragons" fantasy role-playing game, except for the "Legends of Baldur's Gate" mini-series which is tied to the 5th Edition.
A module in "Dungeons & Dragons" is an adventure published by TSR, the term is usually applied to adventures published for all "Dungeons & Dragons" games before 3rd Edition.
 For 3rd Edition and beyond new publisher Wizards of the Coast uses the term adventure.
 For a list of published 3rd and 4th Edition Adventures see List of "Dungeons & Dragons" adventures.
 For description and history of Adventures/Modules see Adventure ("D&D").
 Adventures for various campaign settings are listed in different articles, including Forgotten Realms, Dragonlance, Greyhawk, Mystara, Kara-Tur, Spelljammer, Ravenloft, Al-Qadim, Dark Sun, Planescape, Birthright, and Eberron.
The Dungeons & Dragons Adventure Game is the name of two companion accessories to the second and third editions of the "Dungeons & Dragons" fantasy role-playing game.
 Designed as simpler, stand-alone versions of "Dungeons & Dragons", they feature a simplified ruleset, but with character progression that parallels the standard game.
 However, for 3rd-level characters and higher, the standard "Player's Handbook" is still required.
 The first version was published in 1999 for the second edition of "Advanced Dungeons & Dragons", while the second version was published in 2000 for the third edition of "Dungeons & Dragons".
Thunderspire Labyrinth is the second in the three part series of adventures introducing the new 4th edition "Dungeons & Dragons" concept of Points of Light, a loosely connected and open-ended setting designed to allow modules and Dungeon Masters created materials to be seamlessly integrated into either a single, largely unmapped fantasy world or a Dungeon Master custom made setting.
 The adventure, written by Mike Mearls and Richard Baker was published in 2008 by Wizards of the Coast, as a sequel to the adventure "Keep on the Shadowfell".
 The adventure is designed for character of levels 4-6 and the module code "H" stands for Heroic Tier.
 This module is set in a region of the world called the Nentir Vale, the details of which are given in the 4th edition "Dungeon Master's Guide".
Pyramid of Shadows is the final part of a loosely connected three part series of adventures introducing the 4th edition "Dungeons & Dragons" ruleset.
 The adventure, written by Mike Mearls and James Wyatt, was published in 2008 by Wizards of the Coast, as a sequel to the adventures "Keep on the Shadowfell" and "Thunderspire Labyrinth".
 The adventure is designed for character of levels 7-10 and the module code "H" stands for Heroic Tier.
 This module is set in a region of the world called the Nentir Vale, the details of which are given in the 4th edition "Dungeon Master's Guide".
Keep on the Shadowfell is the first official product from the 4th edition "Dungeons & Dragons" (""D&D"") line.
 It is part one of a three-part series of adventures.
 It introduces a series of 4th edition "Dungeons & Dragons" settings called the Points of Light, a loosely connected and open-ended series of settings designed to allow other modules and fan-created content to be integrated seamlessly into the settings' largely unmapped fantasy world or the Dungeon Master's own custom-made setting.
 The adventure, written by Mike Mearls and Bruce R. Cordell, was published in 2008 by Wizards of the Coast.
 It is followed by the sequels "Thunderspire Labyrinth" and "Pyramid of Shadows".
 The adventure is designed for characters from levels 1 to 3.
 Its module code, "H", stands for Heroic Tier.
 This module is set in a region of the world called the Nentir Vale, which is described in greater detail in the 4th edition "Dungeon Master's Guide".
Dwellers of the Forbidden City is an adventure module, or pre-packaged adventure booklet, ready for use by Dungeon Masters in the "Dungeons & Dragons" ("D&D") fantasy role-playing game.
 The adventure was first used as a module for tournament play at the 1980 Origins Game Fair, and was later published by TSR in 1981 for use with the first edition "Advanced Dungeons & Dragons" rules.
 The module was written by game designer David "Zeb" Cook, who partly ascribes his hiring by TSR to his work on this module.
 In the adventure, the characters are hired to find an object taken to a lost oriental-style city, which has been taken over by a cult of snake-worshipers, the yuan-ti, and their servants, the mongrelmen and tasloi.
 The module was ranked as the 13th greatest "Dungeons & Dragons" adventure of all time by "Dungeon" magazine for the 30th anniversary of the "Dungeons & Dragons" game in 2004.
Dungeon Crawl Classics (DCC) is a series of role-playing game modules published by Goodman Games, most of which use the Open Gaming License (OGL) and System Reference Document (SRD) version 3.5 to provide compatibility with the revised third edition of "Dungeons & Dragons".
 It includes more than 73 adventures, and features celebrated game designers such as Michael Mearls, Dave Arneson, and Monte Cook, as well as classic TSR artists like Jeff Dee, Erol Otus, Jim Roslof, and Jim Holloway.
 The "DCC" series harkens back to early 1st edition "Advanced Dungeons & Dragons" modules in content and style.
Orcus is a fictional demon prince and lord of the undead in many campaign settings for the "Dungeons & Dragons" fantasy role-playing game.
 He is named after Orcus of Roman mythology.
 His symbol is a mace with a human skull as the head.
 Orcus is one of the most detailed demon lords of the "Dungeons & Dragons" game and one of a small handful to be detailed in every edition of the game.
 Orcus was also named as one of the greatest villains in "D&D" history by the final print issue of "Dragon".
 "Dungeons & Dragons 4th Edition for Dummies" describes Orcus as "the most monstrous evil in the game".
Michael Mearls is a writer and designer of fantasy role-playing games (RPGs) and related fiction.