Customized Star Couriers were employed as Sith Infiltrators by Darth Sidious in addition to his apprentices.
The designation "Sith Infiltrator" applied to a variety of starship designs utilized by Sith Masters along with their apprentices during their clandestine operations against the Jedi Order over many centuries. Highly modified Star Couriers were used by Darth Sidious and a couple of his apprentices, fulfilling the role of their Sith Infiltrators. Arguably, the most formidable Infiltrator ever constructed at the time was the Scimitar, which was flown by Darth Maul.
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Star Wars: Complete Vehicles (First mentioned)
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Star Wars: X-Wing Second Edition — Sith Infiltrator Expansion Pack