Invisible Horror

A deceased Invisible Horror. Normally, this is the only way you'll see one.

One of the most unique enemies in the entire Catacomb series, the Invisible Horror is a deadly foe completely unlike any previous foe you've faced.

It appears only in Catacomb Apocalypse.

Under normal circumstances, the beast cannot be seen until it attacks or is killed. When it does reveal itself, it's a massive red beast with the head of a boar and huge claws and tusks.

In Catacomb Apocalypse

Arguably the single most-dangerous monster in the series to date, the Invisible Horror is a foe that cannot be seen with the naked eye, appearing to the player only if it attacks, is hurt, or is killed. They can sometimes be seen, briefly, for a split-second, in a hallway, but otherwise they are completely invisible. These beasts are a very important test of the player's situational awareness, since the one thing they cannot hide from is the Crystal Sphere, even when invisible. The Sphere needs the Red Jewel to detect them, however, so if the player somehow missed the Red Jewel, encounters with Invisible Horrors just became that much harder, and firing down suspicious vectors is going to become your main way of looking for them.

They show up for the first time in The Chamber of Invisible Horror, appropriately enough.

Invisible Horrors like to advance while the player is distracted, usually by other enemies like Sentry Eyes or hazards like Fireballs.

They are formidable melee fighters, but it is not just their invisibility that makes them a threat. Like Nemesis, they are scripted to respond to being hit by performing an attack, which means that if you try to shoot at one when it's up close, it will hit you in retaliation - in addition to melee attacks it will be attempting to do. This means that if you try to rapid-fire one at close range (the usual tactic for dealing with an enemy that gets close), it can deplete your health in a ridiculously short period of time. They even can do this trick (though less efficiently) off Zappers, so under no circumstances should you ever try to rapid-fire one at close-range if you can help it!

Instead, your best bet for dealing with an Invisible Horror that's gotten close is to back away from them and open fire once you clear a little distance, wearing them down with a volley of shots. They have the same stun resistance as a Golden Troll, but they won't be able to do their counter damage and with their moderate health, they aren't too hard to bring down. If all else fails and you have nowhere to run, an Xterminator can potentially one-shot a Horror at close range, so consider that your panic button.

Of course, the best way to beat Invisible Horrors is to never let them get close in the first place. If you have the luck to see one as it's in the distance, that's your invitation to shoot the heck out of it from a distance where it can do no harm. A single Zapper can bring one down from afar, if time is a factor.