Spiritomb is one of those rare Pokemon that has a place on all archetypes of teams, ranging from Stall to pure HO. On a stall team, Spiritomb can function as a bulky spinblocker with its good defenses, which balances the mediocre HP. Although, it does face competition from Misdreavus as the "pure defensive" Ghost-type. Offensive teams utilizes Spiritomb well because they use it to take out pesky Ghost- and Psychic-types, and the priority it offers with Shadow Sneak and Sucker Punch. Defensive sets generally have a hard time due to a lack of recovery outside of RestTalk, and lack of resistances, which is why the offensive sets are more widespread with CB or BlackGlasses.
Its biggest niche from what I've seen is moreso the priority it offers rather than the Dark typing, and it has a way to still hit with Shadow Sneak if the opponent isn't attacking. The Dark-type trapper faces competition from Absol and Drapion, which are stronger and faster, but not as bulky, so pick your poison.
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