Two of Bungie's most popular games join Xbox Game Pass today even though they launched about a decade apart. It's a union of new and old, with the throughline being that they're both beloved.
Let's tackle this chronologically. Halo 3: ODST is available (get ready for the prepositional phrases!) on Xbox Game Pass as part of the Master Chief Collection in conjunction with its launch on PC. This addition is only a recent development for PC. ODST was already on Game Pass on consoles.
However, what's new is the inclusion of Firefight. This cooperative venture was one of the first to help popularize horde modes in the Xbox 360 era. New Halo vanguard 343 Industries brought Firefight back for the PC port, and it'll be included as a free update for the console version too.
You should play ODST. This spinoff started as a small project that kind of snowballed into being one of the better Halo games despite not starring Master Chief. It still gets a lot of love, which is why people have been eyeing this particular Master Chief Collection PC entry for a while now. If Xbox Game Pass isn't your thing, ODST is a $5 standalone purchase on Steam or part of the $40 Master Chief Collection. Both are pretty great deals.
The second game is much more relevant and much hotter and will be downloaded many more times. Destiny 2 and its expansions (including Forsaken and Shadowkeep) hit Xbox Game Pass today on Xbox One. When the Beyond Light expansion launches in November, it'll also be a day one addition to Xbox Game Pass. Microsoft is certainly doing its damnedest to wrestle the title away from PlayStation as the place where you go to play Destiny on consoles.