The Demon is the unofficial name of a horrific demon and the final boss in Silent Hill: Origins, due to the fact that it is not named in the game or in any other official source. Popular fan names include "Flauros" and "Alessa's Dream".
Its design is that of a stereotypical devil, with red skin, long claws and horns, red eyes, sharp fangs and tentacles sprouting from its back. The Demon is probably another pre-incarnate manifestation of the Order's God.
The Demon is given little characterization, beyond its savagery in battle. After Travis Grady completes the Flauros, he intervenes in a ritual being conducted by the Order within the Green Lion Antiques shop. After being sedated by Michael Kaufmann, Travis enters a nightmare-like state where he is forced to battle The Demon, who may have been held prisoner within the so-called "Cage of Peace".
The Demon behaves very similar to the Incubus from Silent Hill. The Demon is quite powerful and should not be taken lightly. It attacks with highly powerful physical moves and demonic magic. It will try to slash at Travis when he gets close and follows him if he runs. The beast can open up its chest for a powerful, beam-like move and also call upon a damaging, flaming rain. It is advised that Travis dodge its attacks carefully and heal when needed. It's best for the player to use their most hard-hitting firearms against it.
It may represent the Order's idea, as a collective, of what God could look like. It may also be Alessa's fear of what God will turn out to be, or it could represent Travis Grady's general perception of the Order and it members.
It may also represent what Alessa thought would be trapped within the Flauros - upon being told by her mother that the Flauros was a "cage for a demon", Alessa envisioned this demon, and the final boss is her perception of what it would look like.
It may represent Travis's own personal demon.
However, according to Silent Hill, and Silent Hill 3, Dahlia has called Alessa the demon, suggesting that this creature might be the manifestation of what Dahlia thinks or sees of Alessa.