Pitiless

Pitiless was a Covenant vessel operated by Kig-Yar. In 2524, the High Prophet of Restraint ordered all Kig-Yar vessels to be searched by squads of Sangheili and Unggoy after the Pitiless was found with "an Unclean being's corpse" aboard.

Trivia

 * While it is not completely clear what is meant by 'The Unclean', it is fairly certain to be either Humanity in general or Spartans in particular due to these Halo Wars timeline entries:

27 November 2525: All SPARTAN-IIs are transported to Damascus Materials Testing Facility on Chi Ceti-4 to be equipped with MJOLNIR armour. They only have a matter of hours to test suit operation before an alien vessel enters the system and attacks their orbiting support ship. In the ensuing battle, three Spartans board the enemy ship and encounter their first Covenant soldiers, capturing an energy shield gauntlet. One Spartan is lost in the engagement.

2525 (Specific Date unspecified, but the entry comes directly after the 27 November entry in the timeline) Hear now the infamy in Chi Ceti system. Due to lack of faith on the part of the crew of the vessel Unrelenting, a band of Unclean were able to board and slay. A Science Lance is ordered formed to study the captured weapons and armour that mitigation action can be taken in the future.