The Terrorbeak is a Shadow Creature with an enormous beak and a slim body with several small legs. It is more dangerous, but less common than the Crawling Horror.
Unlike Crawling Horrors, Terrorbeaks only spawn when the player's Sanity is below 10%. Their Sanity aura will further decrease the player's Sanity by -100/min. Terrorbeaks drop Nightmare Fuel and restore Sanity by 33 points when killed.
Nightmarebeaks are a version of Terrorbeaks encountered in the Ruins (and sometimes in Caves). They are able to attack the player, regardless of Sanity level. They can only spawn from Nightmare Lights/Fissures and do not restore Sanity when killed.
The Terrorbeak moves about as fast as a player with a Walking Cane and cannot be outrun easily. If it is allowed to strike at the player, it will pause for a second to make a threatening animation which can be used to create some distance. If outrun by a great distance, Nightmarebeaks will return to the Nightmare Light or Nightmare Fissure they spawned from. Terrorbeaks, on the other hand, will just materialize closer to the player, as long as their sanity remains low.
Terrorbeaks, like Crawling Horrors, have a "pathfinding" behavior once spawned in. Terrorbeaks keep this behavior going until aggro is drawn.
Terrorbeaks produce a menacing (and sometimes), snarling growl, once aggro is drawn, which is definitely what a player should keep an ear out for. Nightmarebeaks produce the same growl, but with an increased echo and distortion. This audio cue is also used when the Terrorbeak/Nightmarebeak attacks.