Terrors are Summoned like Monsters, with a Summon Terror Action, requiring a Controlled Gate. They are equal to Monsters in their ability to Capture Cultists. That is, Monsters can protect Cultists against Terrors (and vice-versa), and Great Old Ones can still Capture a Cultist protected by a Terror. However, being a separate Unit type, they are not vulnerable to abilities which specifically target Monsters. You may purchase a Terror's Loyalty Card during the Doom Phase.
You may purchase only one Loyalty Card per Doom Phase, so you may not gain a Neutral Monster in the same Doom Phase as a Terror.
When it is your turn to perform a Ritual of Annihilation, simply sacrifice 2 Doom Points and 2 Power. You may still perform a Ritual if you wish. When you spend the Power and Doom, choose a Terror Loyalty Card and place it by your Faction Card, placing the figure on the Map as instructed.
Once you earn a Terror Loyalty Card, you keep it for the rest of the game. From then on, only you may Summon and Control that Terror.