Destiny 2's Ralnik's Hatchet is an item you need to locate as part of a quest.
As you explore the Moon, you'll pick up a variety of 'Essence' quests, with a variety of Moon and Nightmare-related tasks.
One of these is Essence of Jealousy, which you can pick up from the Lectern of Enchantment, with one of the steps is finding the Ralnik's Hatchet within the Traitor's Ketch.
It's a location not on the map, so you might need some pointers of where to find it.
Where to find Traitor's Ketch in Destiny 2
First, let's get to the Traitor's Ketch, which is a location that's technically visible on the map, but is not actually named.
To get there, head south-east from Anchor of Light. On the map this is a giant ship - and it's just as hard to miss when actually playing.
Head inside and once you've made it several rooms in, you'll be at the Traitor's Ketch.
Where to find Ralnik's Hatchet in Destiny 2
Once inside Traitor's Ketch, finding Ralnik's Hatch works a little differently compared to other Essence items.
In this room you'll have to kill a specific enemy for it to drop, named 'Ralnik's Blood of Kells'.
It's a large yellow bar enemy that spawns at the far end, just before the door to the next area. Take it down and Ralnik's Hatchet will be collected straight away.