Teesh

The "Teesh" refers to two different concepts depending on context. First, most commonly, it refers to a sub-sect of the Fated that have their secret headquarters in the Sanctum of the Teesh underground below the Hall of Records.  This sect is currently lead by the planetar, Glavin.  The current purpose of The Teesh is to help the Kateesh in WHATEVER way they can as revealed in Act 2: Information from Glavin. They will go to the most extreme measures to ensure the success of the Kateesh.
The second use of the word "Kateesh" is as a collective term of the missions and people that were selected ("drawn") by the Lady of Pain for various missions throughout history. Each of these missions was a matter of preventing or fixing a major catastrophe in the multiverse that threatened to have such sweeping implications that the multiverse may not have survived without the accomplished mission.
The first of the Teesh, presumably, was the Aateesh. However that was so long ago that history has forgotten anything of what that mission entailed or the people that constituted its party. Indeed we have no idea if that mission was a success or failure and what cataclysmic event may have been prevented or may have occured as a result of that Teesh. The same is true of the Bateeh, Cateesh, Dateesh, Eateesh, etc. They occured simply too long ago to be remembered by anyone other than The Lady and maybe a few elder gods.
However, before the current Kateesh, there have been two Teesh that are remembered by some mortals that still live. The first was The Iateesh. Glavin, Arm of Kord was a member of this Teesh and indeed was the only surviving member of that mission. The others, including his dearest love Nyah (a Dryad of Arborea ) were slain during their struggles. The mission of the Iateesh in 1385 DR (108 years ago) was to discover and prevent the assassination of Mystra, goddess of magic by Shar and Cyric. Unforrtunately The Iateesh failed and Mystra was assassinated by the two evil gods. This brought on the Spellplague -- a 100 year period of great turmoil throughout the multiverse -- and set the stage for The Jateesh.
The most recent Teesh before the current Kateesh was the Jateesh which was drawn in 1463 DR (30 years ago). Their mission was to set extradimensional anchors in the sixteen outer planes so that The Lady of Pain could wrap her chains more thoroughly around them and hold the outer planes of existence on The Great Wheel. This mission was a success and gave The Lady the hold she needed to keep the outer planes from spinning off of the Great Wheel for the next 16 years until the Spellplague came to an end.
Each character of Sigils Sealed learned that not only have they been drawn to The Kateesh, but each has a parent that was Jateesh (Act 2: Information from Glavin).
The fact that the Doors of Sigil all slammed shut at the exact moment when each of the parental Jateesh were planeswalking may not be a coincidence.