The Twenty-One

were a group of twenty-one SPARTAN and pseudo-SPARTAN operators that were secretly recruited for the SPARTAN program to act as the elite special commando teams, intended to support the IVs in the field. All of them were previously deployed in Vice Admiral Serin Osman's secretive 'Black Company', a element of the Clandestine Activities Group that was used mostly as Osman's private force. 
By the time the SPARTAN-IV program approached operational status, the program had already been a major thorn in the side of Osman. The SPARTAN program had recruited numerous IIs and IIIs that Osman had been seeking for her unit, such as SPARTAN-091, SPARTAN-144, SPARTAN-012, SPARTAN-B312, SPARTAN-A259 and so on. They'd also brought in some controversial figures Osman had been seeking to remove, such as Miranda Talavera, Philip 'Katis, David Plissken and CODENAME PAPER. An operation by the 117th also recovered the highly valuable and deadly SPARTAN-000, proving to further damage Osman's position. 
Within Osman's black operations unit existed a large number of Gammas who had served under 013 and 070 during the Great War. These in turn influenced the numerous Sigmas they had been deployed with, creating a number of tight knit units. After a anti-insurgency mission undertaken by both units a number of Osman's SPARTAN units disappeared from their camp and their databases, with the SPARTANS now appearing in the 117th, after a quick and decisive cybernetic attack. This unit was further enhanced by supposed 'rogues' from Tartarus Prison and other areas. This resulted in three elite SPARTAN teams loyal to SPARTAN-013, rather than Osman, as she had hoped. 
Similar such occurrences, by official mandate and internal operations resulted in more SPARTANS, mostly from Gamma class, joining the 117th prior to activation.