Mystara is an alternative D&D campaign world mentioned as a possible starting location for the adventure. It is specifically referenced as the setting containing the city of Specularum, which can serve as a substitute starting point for the adventure instead of the default location of Baldur’s Gate in the Forgotten Realms.
The reference to Mystara appears in the context of providing Dungeon Masters with flexibility in adapting the Tomb of Annihilation adventure to different D&D campaign settings. Along with Oerth (Greyhawk) and Eberron (Sharn), Mystara represents one of several established D&D worlds where DMs can begin the adventure if their campaign isn’t set in the Forgotten Realms.
No additional details about Mystara’s geography, politics, or characteristics are provided in the source text—it serves purely as an alternative setting option for campaign adaptation purposes.