Forest Altars 🪔 can be found on Faulkore Island, which opens at level 14. To reach the first Forest Altar, you must complete the quests "Craft Vivid Flame", and "Collect blossoms".
If you reach a Forest Altar and make an offering to it, it will generate an acolothyst.
You can collect one acolothyst from each Forest Altar.
For more handy tips, check out the articles in the Gameplay section!