You may purchase one neutral unit (plus one Investigator) in each Doom phase.
When it is your turn to perform a Ritual of Annihilation, simply pay 2 Power, then lower your track by 2 Doom points. Then obtain the demon's Loyalty Card, and place the demon at your controlled Gate.
Fourth Circle Terrors act as normal terrors in all ways. However, they all have the "Fourth Circle" weakness, which means that in the Gather Power phase, you lose 1 Power, even if the Fourth Circle Terror is not in play.