If there was one thing we learned about Lyanna Mormont in the brief time we got to know her on Game of Thrones, it was that she would never go down without a fight. As the Battle of Winterfell grew continually more grim thanks to the Army of the Dead's unstoppable advance, little Lyanna gave us a rare moment to cheer about.
In the midst of the action, the tiny, badass Northern leader comes face to face with the Night King's giant, taking him down in the process. She loses her life, but displays a heroism men twice her age couldn't muster. May she rest in peace.
When the undead giant storms the Winterfell gates, he immediately throws Lyanna aside, and it seems like a callous end for a beloved, if small in both stature and import, character. But that's not her end. She gets up and runs towards the enormous beast, he picks her up, crushing her bones in the process. (The sound effects were a bit gruesome, even in context, and it's difficult to listen to Lyanna's agonized screams -- she's just a kid, after all!) But before she gives in to death, she delivers a final blow, straight into the giant's cold, blue eye. She's later revived by the Night King in his massive fuck you to Jon Snow, but we're not going to dwell too much on that, because we're saved the massive ramifications thanks to a well-timed slice of a Catspaw dagger.
She told Jorah, her once-disgraced relative, in the previous episode that she was determined to be in the melee, despite his wishes that she would stay in the crypts. (Though, of course, little did he know that she wouldn't have been much safer, as so many pointed out before the episode.)