Here is my iteration of how it could work.
Swain is launching a massive invasion of Demacia. Part of his strategy is to draw out the statue Galio and have Sion destroy him while he is petrified (Sion's necromantic animation is not enough magic to animate Galio). This would absolutely crush the Demacians' morale, and would ensure a Noxian victory.
Lux is made aware of this plot and, knowing Swain wants to bait out Galio, knows that she's the only one who could get to him and animate him via her own magic. Unfortunately, Swain is taking Lux into account (knowing she's a mage) and sends Talon (or Katarina) to assassinate her before she can intervene. Thankfully Lux is saved by Quinn, who buys Lux the time she needs to escape and reach the battlefront, where Sion is plowing through Demacian forces and is nearly at Galio. Lux gets to Galio just in time and, despite that everyone around can see her, uses her magic to animate Galio, letting the Colossus fight back against Sion.
But Sion cannot be killed through sheer physical might or Petricite weapons. Only magic can truly kill him, and so Garen is forced to protect Lux while she prepares a powerful-enough spell (i.e. Final Spark) to kill Sion. Despite his misgivings on magic, Garen protects Lux because she's family, claiming "You will answer for your deception later", and Lux is able to take out Sion, forcing Swain to withdraw his forces.
What happens afterwards would probably be best figured out by Riot's lore crew.
