Souls of the Lost — ruling 1

The ability that defines Souls of the Lost's power and toughness works in all zones. As long as Souls of the Lost is in your graveyard, that ability will count Souls of the Lost itself.

Souls of the Lost — ruling 2

Cards with the ability word "fathomless descent" have abilities that care how many permanent cards are in your graveyard.

Souls of the Lost — ruling 3

Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.

Souls of the Lost : règles, fonctionnement et interactions (MTG)

Souls of the Lost is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As an additional cost to cast this spell, discard a card or sacrifice a permanent. Fathomless descent — Souls of the Lost's power is equal to the number of permanent cards in your graveyard and its toughness is equal to that number plus 1.

How it works : Souls of the Lost

Original English text — Wizards of the Coast / Scryfall

#1November 10, 2023· Wizards of the Coast
The ability that defines Souls of the Lost's power and toughness works in all zones. As long as Souls of the Lost is in your graveyard, that ability will count Souls of the Lost itself.
No other card is named in this ruling.
#2November 10, 2023· Wizards of the Coast
Cards with the ability word "fathomless descent" have abilities that care how many permanent cards are in your graveyard.
No other card is named in this ruling.
#3November 10, 2023· Wizards of the Coast
Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.
No other card is named in this ruling.