Interestingly, if you activate the later with another character, it will give a dialogue that says "The Altar waits for Someone"...
The "someone" is your main character. Only the main character can interact with them.
These 3 altars connect to God King / the void. There are 2 hints, one is at the deers altar. If you attack/damage the normal ones, you will see them turn to voidwokens also. There are a bit more hint in the dialog if Fane is main.
