It's too rare all right. Manually searching for it for real life days on end and found like 4 stonehenge sites and zero waypointer on both times we've been going after Yagluth. Always found the altar but never the waypoint until way afterwards.

So much gold, tar and Black Metal Ores though..

I had the same problem, got tired after a while and then just used debugmode to fly. Found one after around 20 Minutes.

The vegisir (boss marker) stones for Yagluth can sometimes be found in Fuling Towers and Fuling villages. Also, the Stonehenge structures sometimes have them, as do occasionally just a couple of standing stones next to each other.

As others have pointed out, oftentimes one finds the altar by spotting the distinctive stones of the site. There will be several that look like somewhat like the typical monoliths, except they will be arrange in a circle and the pillars will be curved as they go up, much like a grasping hand. If the weather is clear, they can oftentimes be spotted from quite a distance, so sailing along the Plains in good weather is one way to find them. I have done several run-throughs of this game and more than half the time I find the altar before I find a register stone.

All that said, it can take several hours to finally find Yagluths altar. It can be a pain to sail around for days, looking for it while also checking to spot any Stonehenge or standing stones that may have a register. So hang in there.

Cheers!

I recently found the site, after similar experience. Found it by clearing a henge full of berserkers where there was a stone revealing the location. Best of luck etc. Hopefully your stats are super high after all the searching.

Use this site and you can input your world seed into it to see the map: https://valheim-map.world/

You can then have it show you the boss spawn locations so you can get on with your conquest.

its easier to just find the thing yourself. Its pretty big, you will know it when you see it if you look at a pic online or imagine a skeleton hand the size of a village or a big home base.
Stones are in 2 different locations... the alters and the camps. Both are semi-rare.

also the online map generator can point to both the boss stones and the boss.

Give an honest effort, a couple hours sailing around and exploring Plains.

If you've searched for hours and still nothing then look it up on the Map Viewer: https://valheim-map.world/

We spent about 5 hours sailing today and nothing so far.. Also can't use the map viewer as I'd need the world seed which the devs now hide for deddicated servers to make things a pain.