Nope she already her power from before actually , it sums up to this : when she was young she had two sisters leaders of the freljord tribes , she oneday made deal with this void creatures called watchers in exchange of power , power given to her and her sisters , in exchange of serving the watchers. later she realises these watchers were even more evil than she thought , even her sisters realised before her that these creatures want to destroy the whole runeterra. but the 3 sisters dont want their land to be corrupted by the watchers , so they decide to go on a rebellion in which they fight the watchers on a big war that happened on the bridge of ARAM mode , and lissandra , who realised late the situation she put freljord in , had no choice but either to let the war end and risk seeing her sisters defeated , or end the war herself taking down the 2 teams , Avarosa and serylda will be sacrificed with their armies in exchange of being sure the watchers are going to be sealed in true ice for decades. she chose this option eventually.
then after this big act she is left with a trauma , regretting the death of her sister and fearing the watchers escape their seal , with time she hides her true identity and build the frostguard army in which she is one of the great leaders of freljord , her entire faction is working for a double purpose also : she wants the throne and works in secret to make sure the wacthers seal will not break.
last that happened is that she sent some of her guards to check if the prison is still intact , but they found that its actually soon to break and they returned and warned her , so she must be now losing her shit ,
i like the old lore better , where lissandra seems more badass evil vilain , in the new one she cares more , the old one hated avarosa and knew what she wanted , this one has like second thoughts and regrets that are signs of weakness in my opinion , that doesn't suit an ice witch , cause she is supposedly cold hearted