Sigil is a fictional city and the center of the Planescape campaign setting for the "Dungeons & Dragons" fantasy role-playing game.
David "Zeb" Cook is an American game designer, best known for his work at TSR, Inc., where he was employed for over fifteen years.
The Planescape Campaign Setting is a boxed set for the "Dungeons & Dragons" fantasy role-playing game.
 The set was designed by David "Zeb" Cook and published in 1994.
 It introduced the Planescape setting and was highly praised by "White Wolf" and "Pyramid" magazines.
Blood Wars is a collectible card game produced by TSR, Inc., based on the Planescape campaign setting from "Dungeons & Dragons".
 The game was released in 1995 as part of TSR's 20th anniversary.
 Players use cards representing Warlords and Legions for combat to claim Battlefield cards.
Planes of Chaos was a boxed set for the Planescape campaign setting of the "Dungeons & Dragons" fantasy role-playing game.
The Planewalker's Handbook is an accessory for the Planescape campaign setting in the 2nd edition of the "Advanced Dungeons & Dragons" fantasy role-playing game, published in 1996.
On Hallowed Ground is an accessory book for the "Advanced Dungeons & Dragons" fantasy role-playing game, for the Planescape campaign setting.
Faction War is a book published by TSR in 1998.
 Part of the "Dungeons & Dragons" series.
 It was a published adventure meant for those who were playing in "Dungeons & Dragons"'s fantasy role-playing game Planescape campaign setting, also published by TSR.
The Factions are fictional philosophically based power groups in the Planescape campaign setting for the "Dungeons & Dragons" fantasy role-playing game.
Blood Hostages is a fantasy novel by J. Robert King, set in the Planescape campaign setting, and based on the "Dungeons & Dragons" role-playing game.
 It is the third novel published in the "Blood Wars Trilogy".
 It was published in paperback, January 1996.

Question: Which company was this American game designer who designed the set of Planescape Campaign Setting employed for more than a decade?
Answer: TSR, Inc.