The Molten Spike is a devastating weapon attachment in Mortal Shell, and finding it can greatly enhance your damage-dealing ability.
However, the item is hidden away and will require you to have already acquired at least one sacred gland. When you do have one, the game will introduce a fog and change up the enemy placement. This will happen every time you acquire a gland or talk to an NPC at the hub location that can bring the fog as long as you want if you have a glimpse.
Once the fog is up, locating the Molten Spike is an easy task, you just need to know where it is. Head to Falgrim and find the Grisha mini-boss. Defeat him to get to the chest for the Molten Spike.
Follow the path in the video below, and it will show you where to go and what to expect on your quest to retrieve the item. Just be careful, as the fog introduces a much more aggressive enemy, and there’s lots of them – which is also good for farming tar.