I couldn’t figure out the Elven Ritual segment of these combined quests the first time I was in the ruins, so I went about completing the werewolf quest in the same area first. I’ve since started to come back to side quests. I’ve found and activated the other three tombstones for the quest, and I know the fourth involves the alter and the fountain in the ruins. I follow these walkthrough instructions exactly:

1. Take the earthen jug from the fountain.
2. Fill the earthen jug with water.
3. Leave the pool alone.
4. Place the filled earthen jug on top of the nearby altar.
5. Kneel before the altar and pray.
6. Examine the earthen jug on the altar.
7. Take a single sip from the water in the earthen jug
8. Take the earthen jug.
9. Leave the altar alone.
10. Dump the remaining water from the earthen jug into the fountain.

But when I kneel before the altar and pray, the water apparently bubbles, turns into gas, and when I examine the jug to take a sip, the jug is empty. Since I left the quest with the intention of coming back to it later, does that mean I’ve forfeited it or is there just something wrong with my game? :/

1.Take the earthen jug from the fountain.
2.Fill the earthen jug with water.
3.Leave the pool alone.
4.Place the filled earthen jug on top of the nearby altar.
5.Kneel before the altar and pray.
6.Examine the earthen jug on the altar.
7.Take a single sip from the water in the earthen jug
8.Take the earthen jug.
9.Leave the altar alone.
10.Dump the remaining water from the earthen jug into the fountain.

You need to have the Elven tablet in your inventory when doing the ritual. I made the mistake of selling it and then wasn’t able to complete the quest. If you do have it on you, then i don’t know because the instructions i’ve given are the same as yours.

I hope this helps