WARNING: This article contains spoilers for Halo episode 2.
Paramount+'s fearsome alien Covenant. It's not hard to see the phenomenal creative potential behind these ideas.
Unlike the member races of the Covenant, human beings possess the ability to use Forerunner artifacts. This is because, millennia ago in prehistoric times, Forerunner scientists experimented on some humans, implanting a "geas," or "genesong," within their DNA. It didn't take long for these genesongs to be ed on to others through reproduction, and by the 26th century every single human being possessed the Forerunner geas allowing them to activate Forerunner technology with a touch. Or so it seemed - until Halo 4, which saw a small group of UNSC Marines captured by Jul 'Mdama's Covenant, all apparently unable to use Forerunner artifacts. The issue was never explained.
Paramount+'s Halo adaptation has introduced the concept of the "Blessed Ones." In this new timeline, not all humans possess the necessary genes to interact with Forerunner technology; rather, the ability is rare, the crucial genetic information presumably having been lost over the course of time. To date, only two Blessed Ones have been identified, the Master Chief and the Covenant's agent Makee. Presumably something similar happened in the main Halo timeline, although perhaps there the genesong is more common - shared by John-117, Ellen Anders, Marvin Mobuto, Avery Johnson, and Miranda Keyes. But Jul'Mdama's prisoners did not number among those with the geas.
Halo has been sensible to make the Forerunner genesong a lot rarer. It's a massive change to Human-Forerunner lore, and it raises a lot of exciting story possibilities. For one thing, it remains to be seen whether other Spartans also possess the Forerunner geas, given Dr. Halsey chose her Spartan recruits based on genetic samples she had illegally obtained. This has the potential to create a gulf between the Master Chief and his fellows, or even between the Spartans and the rest of humanity. It's a major difference to the idea seen in the main Halo timeline, but it avoids the continuity problems Halo 4 eventually stumbled into.
Genetic information can indeed be lost with the age of time, so it actually makes sense for the Forerunner geas to be lost from most humans by the 26th century. What's more, if Dr. Halsey was indeed unwittingly looking for the last remnants of the Forerunner genesong when she scoured the galaxy for candidates to the Spartan program, that would explain why the Covenant has struggled to find more than one; because she's accidentally scooped all the last Blessed Ones up. There would be a rich irony if all the Spartans - called "Demons" by the Covenant - were all Blessed Ones in the Halo show.