Expansions introduce new cards, moves, and regions into the TCG scene
Its not legal in any format, but for collectors its a piece of Magic history regardless, forming part of the Power 9 (more on those shortly)
We can recommend them even for people who havent played Magic before, as theyre super casual-friendly, and you get to take your new cards home
Wizards of the Coast introduced them with the Lord of the Rings set in 2023, and theyve appeared in nearly every major release since
I feel Pestilence might be amazing, it's much better if you can give it Lifelink or Deathtouch, usually via a Counter that Nesting Grounds moved (Boot Nipper, Scavenged Brawler, Unbreakable Bond, Unexpected Fangs, Call of the Death-Dweller, Evolved Sleeper, or Void Beckoner, some of those seem solid actually), but even without either it's a great effect to clear out weenies, and your Commander keeps it around pretty consistently