The Son is the embodiment of the dark side of the Force, one of the three beings called the Ones who dwell on the world Mortis, alongside the light-side Daughter and the Father who holds them in balance. He appears in the three-episode Mortis arc of Star Wars: The Clone Wars' third season, beginning with "Overlords", first broadcast on 28 January 2011.
The strangest thing the series did
The Mortis arc drops Anakin Skywalker, Obi-Wan Kenobi and Ahsoka Tano into an outright allegory — a place where the Force has bodies and opinions and a domestic argument. Nothing else in canon Star Wars operates at that register, and the arc has spent fifteen years being either the show's high point or its worst indulgence depending on who is asked.
What he actually wants
To leave. The Son is bound to Mortis by his father and the balance the family maintains, and his entire arc is an attempt to get out into the galaxy where his nature would be consequential. He is not tempting anyone for sport; he needs the Chosen One in order to open the door.
The consequence
He shows Anakin the future — the mask, the Emperor, everything — and Anakin has the memory taken from him before he leaves. That is the arc's most quietly cruel idea: the fall was avoidable, the warning was delivered, and it was erased for the sake of the balance.
Canon status
Canon. The Ones return in Star Wars Rebels and are referenced onward; the Legends shelf's Abeloth, styled a fourth member of the family, has no canon equivalent.