Gargaros

Gargaros (Aki Oni in Japan) is a yokai character in the Yo-Kai Watch series. He is main overseer of "Terror Time", a dimension that children are taken to in their nightmares, and is a physical representation of a person's fears and worries.

He is described as a terrifying, brute of an oni who will hunt down any children that her misbehaving and brutally attack them with his club.

Terror Time
If a child has been misbehaving, such as being after dark, they will be transported to "Terror Time", where they will be relentlessly hunted down by Gargaros. Gargaros represents the child's fears and worries, and if he manages to catch the child he will ruthlessly beat them down with his club. It's possible for child to escape the oni however, particularly by escaping back to their own house. Ordinarily this takes the form of a nightmare, with the child waking up in their bed either after they've escaped, or just after Gargaros took them out. In the case of the former, the child wakes up with a purer mind and soul then before.

In the context of the games, Terror Time occurs as a random event that can spark at any time, mostly at night, when the player in travelling across the map. Unlike what happens to most children, the protagonist is always physically in the Terror Time dimension while he/she is awake, however it is never fully established as to why this is the case.

During Terror Time the player must avoid Gargaros, while also avoiding the other mini-oni that will alert him to their presence if they spot the player. There are various chests scattered throughout Terror Time, however it is possible to simply run straight for the exit back to the normal dimension. As of Yo-Kai Watch 3, a key is first required to be collected before the player can exist Terror Time.

The first time that the protagonist (either Nate or Katie depending on the player's choice), encounters Terror Time occurs when they use Baku in order to sneak out at night to find the lucky undies at the Hot Srping. As they make their way towards the Hot Spring they suddenly get pulled into Terror Time, and run for their life towards the exist from the oni chasing them. After managing to escape, Whisper informs them about what Terror Time is, however he's puzzled over why the protagonist encountered Terror Time while they were awake. However he suggests the possibility that it could be because Baku is currently sleeping in the protagonist's place.

Battling Gargaros
Gargaros is possible to take down in a direct battle doiing Terror Time, and doing so will result in easy access to that Terror Time's chests. However he is incredibly tough and is it recommended that the player doesn't attempt such a thing until at least the post-game. During the first story scripted instance of Terror Time the possibility of defeating him is practically zero, and he can easy wipe out an entire party with a few normal attacks.

If the player does manage to defeat him, his brother ogralus, then Orcanos, will appear during Terror Time instead, both of which are horrifically difficult to defeat, even with a full team of level 99, S-rank yo-kai.

Trivia

 * Whisper's suggested explanation for why Nate/Katie is experiencing Terror Time outside of their dreams doesn't make sense when applied to most of the random occurrences of it. It occurs on nights when Baku is not sleeping in their place, and it can even sometimes trigger during the daytime as well.
 * If the player messes around with their 3DS system clock to change the date and/or time Gargaros will appear when they load their save fiile, calling the player out on it and threatening them. Although there's no direct consequences in the first game, if done repeatedly this will increase the odds of Terror Time occurring. In Yokai Watch 2, doing this will lock the player out of the Crank-a-Kai for 2 days.