Map, that by rumors is needed to place the crystals. So, do that one, and then proceed to next steps.
You can get there by doing soulcrystal ritual (All that boring horde beacon stuff), and firstly, by completing the Medic's Quest, to find the research capsules. He/She gives you a Scorpion-7 Keycard, that you will use to open the door in the laboratory. Then you will see that map.
ALSO THE RADIOTOWER QUEST IS IMPORTANT TOO. MAKE SURE YOU DID IT.

Cursed Pirate Skull is located at bottom level of Liberator, nearby scientists.
Pick it up by clicking F.

Cursed Pirate Bones located at 5 points:
1. The Tribe Camp, that is on the mountain near the Tomsk Farm, it's right on the totem in the centre, go search for that dark rock and somewhere around there is a bone near bushes.
2. Moscow Subway, the bone is here, you will find the subway easily, it's near Arch Of Constantine.
3. Zavod, in the tank, and in the corner, you will try to find it here.
4. Nordwik Farm. The bone is in the well.
5. Camp Kazan, the bone is in the boat near house.

The treasure is near Vladimir Farm, in the water, you can find some pirate shipwreck here, along with the treasure.

After you collect all the bones and skull, you go to the soulcrystal (St. Petersburg, there is a door with a strange sign, that can be opened only at night, type in Area: Anastasia) IT HAS TO BE NIGHT TO TYPE ANASTASIA.
Warning: get a very good gear, it will be hard from then!!!
Then, you place the treasure and pirate skeleton on highlighted spot. After you done that, place horde beacon and start killing zombies near that crystal (you will need to do that again in future, so don't forget).
So, all zombies killed, you teleport to secret Scorpion-7 Arena, and there is pirate boss. I recommend to max some skills, especially ones that increases energy and fasters the sprint.
Completed? Move to next step.

After killing the pirate boss, it teleports you back to soulcrystal, but wait, there is 3 more spawned! Pick them up! You will need it!
Picked up, now go to these locations: Keryev (Place the crystal at highlighted spot, it should be right there when you teleport to it)
Moscow (Below Arch Of Constantine), place the crystal.
And for the final, Yekativurg, you want to place it on the graveyard.

Placed all the crystals? Now, you will need to place horde beacons near these crystals, so you can easily kill zombies NEAR the crystals. It shows you an effect if soul is captured. Wait, what? CRYSTAL DISSAPEARED? LEFT ME WITH THAT BEACON? Do not worry. You can leave now, your main target was just to kill zombies near the crystal. Do that with all of them.
Completed? Great! Next step!

After that crystal killing thingy, you relax and go to charge up the radiotowers.
Remember how you used to repair them for NPC Quest? Like, using broadcast repair kit? Not? Then i'll help you:
1. Radiotower at Mount Nivek, near the Moscow.
2. Radiotower near Vorkuta Junction, there is small shack on small island somewhere around.
3. Radiotower near St. Petersburg, there is a medium island with radiotower.

When you go to radiotowers, instead of repairing them, you charging them, making a long beam, showing the path. You don't go to it, just charge all of radiotowers.

All radiotowers are recharged, you feel proud. Now, everything you need to do is to go to large, white beam appearing somewhere on the mountain, in Moscow. You go there, click F on that and claim the reward. Your long awaited reward, the shadowstalker Mk. II, the best weapon in Unturned.