The Monster is the unnamed antagonist of the horror film "Under The Bed" and is based on the legendary monster under the bed, another name for the Bogeyman - like the bogeyman of legend this monster was a more ghostly, demonic adversary than a traditional monster and more akin to a spiritual evil.
The Monster was responsible for sending the protagonist to an institute for two years after his mother died in a fire, which was blamed on him - in reality the mother died defending both him and his young brother from The Monster.
This would lead to a trouble reunion when the protagonist returned to his family home, meeting his father, his young brother and stepmother as well as a girl he had a romantic interest in and her two obnoxious brothers.
The Monster had began to target the protagonist's young brother in his absence and upon returning the protagonist himself is soon tracked down by the demon - which starts to plague him and his brother with night terrors and poltergeist activities.
Trying to "cure" his sons of what he believed was either insanity or a childhood fear the father sent them to a sleep-over at a neighbors home, with the misfortune of their hosts being the two obnoxious brothers mentioned above.
This, however, angers The Monster and it goes on the berserk, destroying the power in the protagonist's home and then manifesting in the neighbor's home where it snaps one of the obnoxious brothers' neck and pursues the protagonist and his young brother back to their family home.
Once in the family home The Monster commits its next heinous act when it mercilessly and without reason murders the boys' father in front of their eyes in a violent manner - literally tearing him apart as his stepwife screams in horror.
This climaxes in a battle with the two brothers and their stepmother trying to escape The Monster but the protagonist has had enough, after two years of being hounded The Monster pushes him too far when it violently abucts his young brother and he decides to kill the demon once and for all.
With the help of his stepmother he enters The Monster's realm and saves his brother, the three then race out of the family home and The Monster manifests in the garage - where the protagonist tries to kill it with a chainsaw but it overpowers him.
This results in the protagonist shattering the urn holding his mother's ashes and the demon recoils as her ashes burn it (a symbol of her love for her children) - the protagonist thus tosses his mother's ashes at The Monster, which ultimately cause it to burn away and he embraces his stepmother and brother as a police van arrives at the home.. suggesting the protagonist will likely be put back in an institute due to the deaths that occured.. however his young brother has hope now, free from The Monster.