Conversation so far:
What was the Hellfire Club?
Instead of being chosen for the first X-Men class, she is instead sent to spy on the Hellfire Club as Tessa.
Why was she sent to spy on them?
Years later, Sage meets up again with Xavier, who at the time is recruiting the original X-Men.
Instead of being chosen for the first X-Men class.
Why was he recruiting  them?
CANNOTANSWER.
Was he successful?
While she is able to successfully warn Psylocke away from the Hellfire Club, Tessa is later remorseful.
Why was she remorseful?
she is unable to similarly prevent the corruption of Phoenix into the Black Queen.
This transformation is a key factor in the creation of Dark Phoenix.

Question: So she created Dark Phoenix?
Self-contained rewrite: So Sage (Tessa) created Dark Phoenix?