Gustaf Johansen tiene una Respuesta.
Gustaf Johansen
The Harbinger
Type: Character Faction: Cthulhu
Cost: 1 Skill: 1 Icons: (C)
Game Text:
Cultist.
Forced Response: After a Cultist leaves play, draw 1 card. Limit 3 times per turn.
Flavor Text: “There’s no stopping it! We must prepare for the inevitable.”
Set: The Sleeper Below
Number: 003
Illustrator: Julian Kok

FAQ escribió:Response effects are played after the resolution of the action or framework game event that meets their play requirement,
Las Respuestas se aplican después de haber resuelto la acción que las "activó". En este caso, después de que se usara la Piedra de la Plaga, por lo que Gustaf Johansen ya estaría en la pila de descartes. Sí está en la pila de descartes, entonces su texto ya está en blanco y no puede activarse
Una anotación importante de la FAQ
FAQ escribió:Responses on Cards Leaving Play
Some cards respond to themselves leaving play, self-referentially, or to effects that may cause that card to leave play. Such responses can be triggered as if the card were still in play. Note that only Response or Forced Response effects can be triggered in this manner, and they must respond to leaving play or the effect that causes them to leave play. Cards may not take “one last Action” before they leave play
Es decir algunas cartas tienen una respuesta del estilo "Respuesta Cuando esta carta entre en la pila de descartes entonces....". Entonces sí que se puede activar su respuesta desde la pila de descartes.
El ejemplo que pone la FAQ es: Professor Nathaniel Peaslee
Professor Nathaniel Peaslee escribió:Professor Nathaniel Peaslee
Scholar of the Arcane
Type: Character Faction: Miskatonic University
Cost: 4 Skill: 3 Icons: (C)(C)(A)(I)
Game Text:
Investigator
Response: After
Professor Nathaniel Peaslee enters your discard pile from play, pay 2 to put into play all characters that entered a discard pile this phase.
Set: Core
Number: 24
Illustrator: Felicia Cano
Roald Ellsworth sin embargo es una
InterrupciónRoald Ellsworth
Intrepid Explorer
Type: Character Faction: Miskatonic University
Cost: 3 Skill: 2 Icons: (A)(I)
Game Text:
Explorer. Faculty.
Loyal.
Explorer characters gain Willpower.
Disrupt: When an Explorer character you control would leave play, draw 2 cards or put 1 card from your discard pile into your hand.
Set: SoK
Number: 4
Illustrator: David Chen

De la FAQ tenemos el siguiente orden:
FAQ escribió:1. Action is initiated.
2. Disrupts.
3. Action is executed.
4. Passive abilities (requirements now met) are initiated
Tenemos la Interrupción entre la inicialización de la acción y su ejecución. Tenemos exactamente en que punto Roald se va a la pila de descartes
1) Action is initiated
After a player initiates an action, the timing window starts.
For the initiation stage of any player action, a player must go through the following sub steps, in order. The first step is always revealing the card or declaring the intent to use an ability.
Then:
a) Determine the cost (to either play the card or pay for the card’s effect) or costs (if multiple costs are necessary for the intended action).
b) Check play restrictions, including verification and designation of applicable targets or cards to be effected. c) Apply any penalties to the cost(s). (Any effects that modify a penalty are applied to that penalty before it becomes a part of the cost.)
d) Apply any other active modifiers (including reducers) to the cost(s).
e) Pay the cost(s).
f) Play the card, or trigger the effect, and proceed to step two.
3) Action is executed
The active player now executes the effects of the action. If this action discards or destroys one or more cards, returns one or more cards to a player’s hand or deck, these cards immediately leave play
Es decir, Roald abandona el juego en el punto 3... y por ser una Interrupción se activa en el punto 2. Yo considero que Roald Ellsworth se puede activar a si mismo.
Espero no haberla cagado.