It stays near the ruins in the Crimson Mirelands. If you want to change its gender, return to the village, and come back. After you catch it, there is a VERY low chance to find another Spiritomb in front of the ruins instead of a Pokémon that spawns there normally. There is a 0.5% chance for Alpha Spiritomb. You can only encounter Spiritomb Mass Outbreaks or Mass-Mass Outbreaks after you complete the questline Eerie Apparitions in The Night, and catch Spiritomb the first time. Hope this helps!
Source: AustinJohnPlays wisp guide videos