After taking the quest, talk to Koris again, and he will take the party into the Secret Lair of the Snowmen dungeon. You will encounter multiple large groups of three kinds of snowman golems as you make your way, in order they are; Glimmering Snowman, Mischievous Snowman, and Industrious Snowman, a snowball throwing contraption and 'jets' of cold shooting up from the floor that do damage. Finally, you will come upon the Monk boss, Freezie. After defeating him, continue on and discover the "treasure". When all party members have collected their prize from the Chest of Wintersday Past, you will be taken back to Umbral Grotto and able to claim your reward.