Denise Crosby has teased her seemingly impossible return to the role of Tasha Yar in Star Trek: Picard season 3, but the possibility of Yar's return has already been set up by Star Trek: The Next Generation. During an appearance at the 56-Year Mission Star Trek convention in Las Vegas, actor Denise Crosby revealed that Tasha Yar would return for Star Trek: Picard season 3's TNG reunion, but refused to be drawn on how exactly her character would return. Denise Crosby was one of the original Star Trek: The Next Generation cast members but left the role of Tasha Yar toward the end of season 1 due to a lack of interesting storylines for her character.
In Star Trek: The Next Generation season 1, episode 23, "Skin of Evil", Tasha Yar was killed by the alien entity Armus during a rescue mission on the planet Vagra II. Despite Tasha Yar's death, Denise Crosby returned several times across the course of TNG's seven seasons. As well as reprising the role of Yar, Crosby also played Yar's half-Romulan daughter Sela in four episodes, first appearing in season 4, episode 24, "Mind's Eye". It's Sela's origins that could hold the key to Tasha's miraculous resurrection as part of Picard season 3's TNG reunion.
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Sela was the product of a relationship between a Romulan general and a version of Tasha Yar from an alternate timeline created in TNG season 3, episode 15, "Yesterday's Enterprise". In that timeline, the disappearance of the Enterprise-C through a temporal rift continued a decades-long war with the Klingons. Discovering that, in the prime TNG timeline, Tasha Yar died an empty death, the alternate version makes a heroic sacrifice by joining the crew of the Enterprise-C as they return to their time to be destroyed by the Romulans, an honorable death that would inspire peace with the Klingons. However, the fate of the Enterprise-C was purely a hunch by the alternate universe version of Guinan (Whoopi Goldberg) and TNG didn't confirm or deny what happened to this Tasha and the Enterprise-C until her daughter Sela appears in season 5, episode 1, "Redemption II".
It's established in Star Trek: The Next Generation canon that Tasha and a handful of Enterprise-C officers survived their battle with the Romulans on returning to the prime timeline, but were held as prisoners of war. In an attempt to save the lives of her surviving crew mates, Yar agreed to become the consort of the Romulan general. Becoming pregnant with Sela, Tasha raised the child for four years before attempting an escape, according to Sela, she was caught and subsequently executed. These events are related to the audience in "Redemption," a story that Star Trek creator Gene Roddenberry tried to cancel.
The events leading to Tasha Yar's second death are recounted to Picard (Patrick Stewart) by her daughter Sela, an inscrutable Romulan officer and an unreliable narrator who's in the middle of a negotiation with a Starfleet captain, it could easily be a story designed to gain the upper hand. Not only that, but Sela was four years old at the time of her mother's execution, a traumatic event for a child to witness. This, combined with the continued Romulan presence through both seasons of Picard leaves season 3's writers plenty of room with which to enable Tasha Yar's Star Trek: Picard return.