This page explains how to encounter Spiritomb in Pokémon: Brilliant Diamond & Shining Pearl.
Where to Find the Odd Keystone
To encounter Spiritomb, players must obtain the Odd Keystone. This is gotten by speaking to a Black Belt NPC on Route 208, hidden in the bottom-right corner, near a Honey Tree. It’s also near the gatehouse to Hearthome City.
Where to Find Hallowed Tower
After obtaining the Odd Keystone you must then travel to Route 209, east from Hearthome City. Upon progressing through the route you will stumble upon an odd-looking structure known as the Hallowed Tower, and be prompted to place the Odd Keystone inside. This is where things will get tricky!
How to Encounter Spiritomb
Once the Odd Keystone is placed in the Hallowed Tower, players must then venture into the Grand Underground, where they must encounter 32 or more unique characters.
The wild Spiritomb can attack using STAB Hex and Payback. It can also boost its Special Attack using Nasty Plot, which can make it scary if the battle drags on. Besides that, it can use Spite to lower your PP.
Due to its elusive typing, most attacks are neutral towards it. Normal-type moves obviously have no effect, while Poison does half damage. Avoid Fairy-type moves, since those are super-effective and could cause you to accidentally KO Spiritomb.