The Ancient Altar, which is used for praying to The Huntress, is located through the magical green barrier in the River Tunnels under the town of River's Rest, near the magical blue barrier used to teleport to the Citadel.
Wonders of Elanthia Historical Discussion
"I well recall my first venture into these River caverns. Yes, now... come along, one by one. It's a bit narrow in here. You see, as a young fellow, I had a bit of trouble with the local jungle's trolls. Namely, I discovered myself prepared for their stewpots one day after an otherwise pleasant jaunt. My, I hate trolls. Near on the edge of death, I made my way here, for I'd heard rumors here of an altar to vengeance. A magical green barrier guards the entrance to Her shrine here, and I had not yet discovered the secret of passing through it. So I prayed to The Huntress and railed at Her at the same time, for She seemed not to hear my calls for vengeance against the trolls that had so nearly ended my brief existence. And, like a fool, I chose to seek vengeance against them on my own. I returned to seek vengeance against the trolls. It did not go well. They took my hand, first, and forced me to watch as they turned each and every finger into stew. It was their plan, I presume, to keep me alive for days, and use me for food bit by bit. Yet as they rested, something wonderful and terrible began to occur. First, it was just a bubble of activity on the back of one troll. Then a twitch here, a chance flexion there. Within minutes, however, I saw The Huntress's hand at work. Limbs began to erupt where there should be none. Organs clearly out of place began to sprout from their very skin. She had turned my lost hand into a weapon, a poison that sent their prized regenerative abilities out of control. And so I left them there, undying but unable to move. I can only imagine that Her merciful hand ended those creatures eventually...either that, or we've got some blubbery, gelatinous troll-creatures oozing about."
Entering the magical green barrier
The magical green barrier is normally closed, and can only be opened magically. In order to open the barrier, a number of first-level spells must be cast at it from different professional base circles. Although there may be several successful combinations, apparently Prayer of Holding (301), Natural Colors (601) and Blood Burst (701) is a sufficient combination, as shown below.