During each Doom Phase you may purchase one Investigator, plus one other neutral unit.
To purchase an Investigator, simply pay 1 Doom when it is your turn to perform a Ritual of Annihilation. If you wish, you may still perform a Ritual. When you purchase an Investigator, give its Loyalty card to a player who does not yet have an Investigator. That player now controls that Investigator for the rest of the game.
A player can never have more than one Investigator card. (You CAN give an Investigator to yourself, in order to avoid a worse fate!)
Investigators count as Cultists, but they do not provide Power and cannot Build or Control Gates. They can be Captured or Recruited like any other cultist.
Investigators' abilities are never optional; they must always be applied.