Merrin is a Dathomirian Nightsister, sole known survivor of the massacre of her clan, introduced in Star Wars Jedi: Fallen Order (2019) and returning in Star Wars Jedi: Survivor (2023).
Not a dark sider, exactly
She belongs in a Sith and dark side catalogue and she is not remotely a Sith. Nightsister magick is a separate tradition with its own techniques — the summoning of the dead among them — and Merrin is the clearest canon demonstration that the dark side is not the same thing as the Sith Order. This site catalogues her here because the Nightsisters are where that distinction is made, not because she serves anybody's empire.
The lie she was raised on
Her clan was destroyed by General Grievous during the Clone Wars, an event dramatised in The Clone Wars' "Massacre" (2012). The fallen Jedi Taron Malicos, hiding on Dathomir afterwards, spent years telling the surviving child that the Jedi had done it. She was a small girl and there was nobody else alive to correct him.
Resurrection
She can perform the Chant of Resurrection, the Nightsisters' most demanding working — a capability the material otherwise attributes only to Old Daka. That is a substantial thing to hand a survivor: the one power that could theoretically undo what was done to her clan, and no bodies left to use it on.
Canon status
Canon. The Nightsisters originate on the Legends shelf in Dave Wolverton's The Courtship of Princess Leia (1994), but Merrin is a 2019 canon creation and the two versions of Dathomir differ substantially.