If you want to get Spiritbomb in Pokémon Legends Arceus, you'll first need to complete a side quest that you'll unlock during the main storyline, when you've calmed down Chopper. Thus, going to rest at the Logis, you will meet Pitris who will tell you about a mysterious evil, flames being scattered in Hisui and who will give you a Keystone .
As you will have understood, your objective will be to find the 107 Strange Flames and collect them to absorb them into your Keystone. Once all the flames are gathered, you will be able to capture Spiritomb . Below is an image and the exact location of each flame, region by region. In reality, there are 108 but the very last flame will be in front of Pitris.
All Strange Flames in the Obsidian Plains
There are 20 Flames to find in this area.
Rusti-Cite
There are 7 flames hidden in the village. The first is the one you get when starting the quest.
The Flames at Crimson Swamp
There are 20 flames to find in this region.
The Flames at the Lazulli Coast
There are 20 flames to find in this region.
The Flames at the Crowned Foothills
There are 20 flames to find in this region. Some are hidden in caves and caverns so be sure to return.
The Flames of the Pristine Lands
There are 20 flames to find in this region.
Capture Spiritomb
Once the 107 flames have been collected, bring them back to Pitris and accompany her to the Misty Ruins located in Crimson Swamp.
Collect the last flame in front of the stele. Remember to take good Pokémon in your team before anything else because you will have to face Spiritomb. Also take Balls to capture it.
- Reward : 3 Double Bite and Spiritomb appears behind you. He is level 60.