Archivum Data Disc

The Archivum Data Disc starts the Algalon the Observer attunement quest chain in Normal mode Ulduar.

Source
This item is a reward for completing the Assembly of Iron encounter. Only one player can acquire it per lockout period.

The disc is a guaranteed drop so long as Runemaster Molgeim or Steelbreaker is killed last (Medium difficulty and Hard mode, respectively).

As a quest objective
This item starts Archivum Data Disc (quest).