Lissandra, Avarosa, and Serylda were three Iceborn sisters who served the Frozen Watchers until Avarosa decided to fight for independence, which Lissandra saw as betraying the ones who gave them everything in exchange for 'small' things.
Lissandra murdered Avarosa (Serylda's fate is unknown). Centuries later (and knowledge of the Watchers completely erased by Lissandra's hand) their descendants (Ashe and Sejuani respectively) are in open civil war for the future of the Freljord, the perfect moment for Lissandra to strike and pave the way for the Watchers' return.
Her influence (Dark Ice corrupting the land) was felt by Anivia, Udyr (who even met her personally) and Volibear, who decided to take action.
Lissandra has found an uneasy ally on Trundle and the Ice Trolls (the King may betray her at any moment).
Lissandra was the 'Seeker', the intermediary between the Iceborn and the Watchers. The Seeker's Armguard used to be hers, but it's now in Sejuani's possession (how the Winter's Wrath got it is uncertain).
Currently Lissandra is the leader of the Frostguard, and is masquerading as an ally of Ashe's and the Avarosan's against Sejuani and the Winter's Claw, all to strike at the Frost Archer's tribe when they least expect it.
Lissandra met with Quinn and Valor in the Frostguard citadel when they journeyed to the Freljord on a scouting mission at Jarvan IV's behest.
She later pursued them in her true form (the Ice Witch) after they sneaked out the citadel (neither knows both incarnations are the same person).
Lissandra is being targeted by Kalista for betraying the Iceborn and murdering Avarosa.
↑Beekess, R.S.P. Etymological Dictionary of Greek, p. 103-4, 881