https://universe.leagueoflegends.com/en_US/story/champion/senna/
Senna first encountered the Mist at an early age, when wreckage from a distant Harrowing washed onto the shores of her home island. The Black Mist within the wreckage awakened upon contact with life. She and her village survived the ensuing storm of souls, thanks to the intervention of a nearby Sentinel… but in the wake of the attack, the Mist was still mysteriously drawn to Senna.
She was cursed, marked by the Mist so its horrors pursued her endlessly, the darkness drawn to her like dying moth to living flame.
She learned that life had been the origin of her curse. Its spark shone brighter within her than in anyone else—she’d been infected with it when she first encountered the wreckage that brought the Harrowing. There, she’d been touched by a powerful, lingering soul, given its unnatural life…
It was life that the Black Mist could never let go.
When Lucian drove his broken pistol into the lantern, intending to end the torture of the souls within, Senna was waiting. She escaped, shrouded in Mist she’d drawn from other souls. She was dead, but also alive, thanks to her curse, wielding a relic-stone cannon that could channel darkness along with light, forged from the weapons of fallen Sentinels.
- Senna being "alive": _She learned that life had been the origin of her curse. ... There, she’d been touched by a powerful, lingering soul, given its unnatural life… It was life that the Black Mist could never let go. _
- The origin of her weapon: ...wielding a relic-stone cannon that could channel darkness along with light, forged from the weapons of fallen Sentinels.
It seems like the Lantern is almost like a new Realm (of Runeterra), because it can trap spirits, but has to be material to some degree. Physical things like corpses and their belongings seem to be in it as well, allowing Senna to build her weapon.
For her abilities: It seems like Q, W & R are her Sentinel training (compare to Lucian's Q, W and R respectively), but enhanced with the life, and the mist. This is how she can heal & shield, and W can root because of the Mist.
Her E is special, presumably she learned to control it in the lantern: She escaped, shrouded in Mist she’d drawn from other souls.
