I completed MCC 2 days ago on legendary to get the “legend” achievement. I got the achievement, but there is no helioskrill to be found. Is something wrong with this system?
Sometimes just takes a bit. Try changing a customization item in MCC like an emblem or an armor piece so it forces a sync. Optionally, do a hard reset of your Xbox and open up MCC again. Once you’ve done the first or both, boot up Halo 5 and you should see Helioskrill there.