Frostfall

 is a mace in.

Steps

 * Acquire Betrayal, from The Endless Queries.
 * Acquire Waidwen's Sundial, from The Bridge Ablaze.
 * Approach the entrance to The Drowned Kingdom. You will be attacked by the aumaua spirit (Shambling Soul / Ghostly Aumaua (convo)), after it reverses the water flow. Kill the spirit.
 * Go to the platform just above from the aumaua spirit. You can now cross the water.
 * Talk to the kuaru spirit. She's busy fixing a bridge, and won't let you pass unless you pose as her ranga, using Betrayal.
 * Use the gateway to teleport to the east-southeastern part of the map.
 * Use Waidwen's Sundial to activate the ray of light to repair the bridge. Cross the bridge and talk to the Glamfellen Soul. Depending on you dialogue choice, she will either give you the mace, or you take it from her.

Upgrades

 * Note: Chaos Strike / Dispersed Suffering and Encroaching Frost / Shattering Head are mutually exclusive.