The Helmsman leads the armada
The armada contained at least dozens of Star Destroyers of various classes, as well as hundreds of smaller warships, seemingly gathered from various peripheral naval forces. Leading the fleet, was the Helmsman, a Praetor Mark II-class battlecruiser commanded by Giel himself, and also the location of the Teezl's transport cage. At least one vessel of similar design was also part of the fleet.
When gathered at an undisclosed location near the galaxy's edge, the fleet numbered enough ships to be noted as the biggest in Imperial history, even eclipsing similar units like Darth Vader's Death Squadron.
The mysterious Teezl being maintained inside its containment cage.
In 3 ABY, a bizarre creature known as the Teezl was discovered in a remote region of the galaxy. The creature had the unique ability to send instantanous messages across vast distances of space, and the Galactic Empire soon realized the enormous potential of using the creature as a communications hub for Imperial forces spread throughout the galaxy.
To protect the creature on its journey to Imperial Center, an Imperial decree went out, authorizing the secret assembly of a gigantic fleet to transport and protect it. Giel was assigned the task of leading the armada, as well as supervising its creation.
As the armada prepared to depart for the Core Worlds, a Rebel strike-team raided the Spindrift Station, an Imperial outpost that held the fleet's route coordinates. Using the information to plan an attack, the Alliance decided an all-out attack to be too costly.
The infighting escalates as a result of Rebel infiltration.
Instead, four customized TIE/LN starfighters were prepared for an infiltration mission, into the heart of the armada. With specially charged laser shots, the fighters would sow confusion amongst the ships of the fleet, allowing them to slip past the defenses in the ensuing chaos and firefights and destroy the Teezl with an extra-powerful blast.
Operation Earplug went reasonably well. With the loss of two pilots and the withdrawal of a third, Commander Luke Skywalker finished off the Teezl as planned and escaped pursuing TIE fighters after he was found out.
As a result of the catastrophic failure, Giel was demoted to Lieutenant, a relatively light punishment given by Darth Vader, who would usually kill his officers for minor offenses.