Firstly, you have to complete the quest Pirates of the Abyss, doing so you have to find 5 pieces of pirate map fragments and then you could discover the pirate base.

Once you have plundered the pirate base and obtained the Chest of the Drowned, a legendary relic of the pirate goddess Besmera, it will contain random loot for every single playthrough ever since.

Then take a look inside the chest for its content, if you are lucky (chance 20%) you could get the Holy Symbol of Lamshtu from it:

If you are unlucky you have to give up the current playthrough and check the chest in a new one.

If you got the holy symbol, then you have to inspect the socket on the wall of the house in docks

and pass the Lore (Religion) check DC 44 (on unfair, so probably lower on other difficulties)

Then a portal appears and you can enter

 but inside is nothing but a slain corpses and dry fountains which are no use for you at that moment.

Then you play normally and pay an eye for corrupted alters and special islands, namely those "cave-with-sand", "fire-flying-rocks" and "techno" islands.

Also pay attention to Skeletal Salesman and if you meet him, buy the Small Sextant from him or buy it back from Greedy Chest if you stored it there in previous run.

If you are lucky enough, mostly after you kill the Archpriest so islands 45+, you may discover corrupted altar on the fire island where are flying rocks all around:

And the hard part comes, every alter is different and it needs different ritual to unlock.

For the very first one you need the Small Sextant and Scroll of Flame Strike or a divine caster with prepared Flame Strike spell.

You put the sextant and scroll of Divine Favor into the altar.

Then you cast Flame Strike from a scroll or as a spell upon the altar.

I’m not sure if you have to hit yourself as part of the ritual and sacrifice a little of your blood or not, but I did and it worked.

Then you have to go closer to the altar to pass the perception check DC 34 to spot a mysterious scorched runestone

Pick it up and for later use

 if you leave it there, you cannot complete the quest, so it is really important

Because you could lose the runestone on death, so as you return to the city, go directly to the abandoned house and use the runestone to fill the dry fountain.

Then continue you journey until you venture on an island with Wild Magic undertow, which is important, meanwhile buy a new sextant from skeletal merchant (I bought two and transfer them between playthroughs by selling them to Greedy Chest.

On Wild Magic Island you have to summon Deathsnatcher, to do so, you have to use spell which targets the ground. The whole process is random and it could take some time, so kill the monsters around first not to be disturbed.

When you succeed and successfully killed the demon, loot his corpse for Crystalized Wild Magic Fluid

Then you have to be lucky again and find a corrupted altar. Don’t forget to transfer the crystal by selling it to Greedy Chest when you are about to finish the playthrough or you have to repeat the process for getting one.

The problem is that the caster has to fail the concentration check, so increase your game difficulty if you are not already on unfair to fail that on purpose.

Meanwhile you could encounter this strange obelisk which is not for a blessing as usual on special underground island:

But if you inspect it that you smell strange chemical odor.

Use any acid spell, cantrip will be enough and hit it with that. Then step closer and pass DC 34 perception to spot the Mysterious Melted Runestone.

Pick up it and that’s all for now.

After you get back to the city, place the runestone into the dry fountain

And the last runestone needs to be obtained

If you missed your runestone island, don’t worry, the runestones are on the islands with certain configuration and they are cycled so you have chance to visit the right island with the runestone in time.

Pay attention on techno islands for the last runestone. You need to spot this mysterious device:

Place the acquired Crystalized Wild Magic Fluids inside

Now you have to energize the device by electricity and you could use the Jolt cantrip to do so, but any electricity spell should do the trick

After an explosion you should see the Mysterious Dented Runestone, so pick it up for the last Dry Fountain

After you return to the city go to the Abandoned Warehouse and place the runestone into the last Dry Fountain

Let’s have fun with Nahyndrian Darkness!

There's a little secret regarding Nahyndrian Darkness: 

Keep an eye on the glowing crystals, they could be destroyed during the combat, the first green is for spell immunity and once broken you could cast spell on Nahyndrian Darkness and the second blue is for summoning and teleportation; beware though that the destroying of the teleportation crystal allows Nahyndrian Darkness to summon her brethren Playful Darkness and Inevitable Darkness to help her, so even after her death when crystals explode  they will come and you have to fight them regardless.

Fighting her will be a lot easier if you are Lamashtu's follower 😉