Mirrorwater was the first Factions storyline. It was followed by Ceruleana.
A strange comet fell from the heavens and crashed into neutral Council territory. At the crash site was discovered a magical silver pool. The tribes of the Freljord briefly united to enter the League and win control of the Mirrorwater, which was divided up among them after their victory.
The Freljord ultimately prevailed. The tribes of the Freljord divided up the Mirrorwater amongst themselves. The Avarosan used it to build a fortress, the Winter’s Claw fashioned it into weapons and armor, and the Frostguard (behind a cover story about Trundle’s trolls drinking it or somesuch) poured it into Lissandra’s underground ice pools to feed her world-changing magic.