This is an unmarked miscellaneous quest. The title "The Flask" will not appear in game anywhere.
After the player completes the quest Something Strange, the Guide Spirit may tell the player they remembered something and would like to speak to the player.
Asking the Guide Spirit, “You wanted to talk about something?” will have her explain that she recalls an item that could be of use to the player. The player can either decline to find the item or agree to search for it. Both paths will unlock the possibility to find the Spirit Flask item in a small strongbox hidden in the hub, however only agreeing to find it will give the player an objective reminding them to search for it.
Retrieving the Spirit Flask involves completing a small puzzle in the hub. The objective points the player to a side room in the hub - in which the marker disables when the player nears.
To retrieve the Spirit Flask, the player must - in this order:
If the player gets the order wrong, the Pillars reset and the player must try again.
Once the order is correct, the Soul Gems on the Pillars will generate some VFX pointing towards a small strongbox which will then contain the Spirit Flask. Upon retrieving the flask, the Guide Spirit will make a few additional comments on it if able and the quest will complete.