Spooky Message

You get Spiritomb at the conclusion of part 2. Part three you need to do something (forget what, but not particularly difficult). For that you get 18 Spiritomb candy and 1080 stardust.

Part 3/3 is similar to parts 8/8 and 9/9 for the Mew/Celebi/Meltan researches. There aren't any tasks to be done, you just claim them for some extra exp.