What it does
Fathomless descent isn't an official keyword—the actual mechanic is Descend, introduced in Lost Caverns of Ixalan. "Fathomless descent" likely refers to descend abilities, which care about how many permanent cards are in your graveyard. There are two flavors: Descend X (rewards you if your graveyard has X or more permanent cards) and descend on a count (effects that scale with your graveyard's permanent count).
The nuance people miss: descend only counts permanent cards—creatures, artifacts, enchantments, lands, planeswalkers, battles. Instants and sorceries in your graveyard do not count, even though they're sitting right there. Also, "the fathomless descent" is the named mechanic referencing this graveyard tracking on cards like Spelunking.
In Commander, descend rewards graveyard-fueling strategies—self-mill, fetchlands, sacrifice. It synergizes with reanimator and landfall decks, giving extra value for filling your yard, though you'll want plenty of permanents (not spells) to hit higher thresholds reliably.
Iconic cards with Fathomless descent
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