Planescape: Torment

Planescape: Torment in one of the sister games of the Baldur's Gate series and the Icewind Dale series. It was developed by Black Ilse Studios and was released on December 12 1999.

Unlike the other games in the family, PS:T takes place in eponymous Planescape setting, which is - unlike Forgotten Realms - not based on medieval fantasy but instead incorporates a vast number of different genres (such as steampunk, fantasy, etc.). The nature of the Planescape setting effectively connects all Dungeons & Dragons settings with each other.

In-game Crossovers
Baldur's Gate and Planescape: Torment feature a number of crossovers and cross-references.

Notable setting crossovers:

 * Haer'Dalis is a tiefling, a race of fiendblooded humanoids who originated in Planescape. While realmborn tieflings do indeed exist, he is actually from Sigil, which is the center of the Planescape setting and also the main location in PS:T.
 * The Blood War is referenced in Baldur's Gate II: Throne of Bhaal.
 * The Planar Sphere in Baldur's Gate II is a device that allows interplanar travels.

Game specific crossovers:

 * The collector's edition of Baldur's Gate II and BG2:EE feature a bonus merchant named Deidre, who sells PS:T themed weapons and gear.
 * Aphril - one of the inmates in Spellhold - can see through planes, which includes a two instances of her seeing things from PS:T.
 * Sigil and the Lady of Pain: "I see... a city of changing streets under the Lady of Pain. My eyes... pluck out my eyes."
 * The Nameless One in front of the pillar of skulls in Avernus: "I see... a walking corpse... he speaks... to a pillar of skulls? See, it is madness!"

Links

 * Torment Wiki