Spawn of Ithaqua

The Spawn of Ithaqua (previously Spawn of the Winds) are half-breed offspring of Ithaqua and a human. They usually appear human, but can transform into creatures more akin to their father.

In the RPG...

Despite appearing human before their change, the Spawn of Ithaqua do not have regular human stats. SIZ, INT, and APP are the only ones that are. All the rest are a fair bit higher, though DEX is the only one that's average is above human range. Once the transformation takes place, their "monstrous form" really is monstrous, with their SIZ increasing by 5 times! STR, CON, and DEX also increase, while INT and POW remain the same. The Spawn basically becomes a deity at this point, with 55 hit-points, 6 armour, and four attacks per round that each average 21 damage.

    Almost every stat for both human and monstrous forms is different in the 7e Malleus Monstrorum compared to the old edition, so I'm not going to detail them.

    The Spawn of the Winds are also able to control the weather—expending Magic Points to do so.

    It doesn't specify in either edition of the Malleus Monstrorum whether the change is permanent. It says the initial change can occur by choice or by anger, but doesn't say if they can return to human form afterwards... like a Mythos version of the Hulk.

In a video game...

Even though they are human by default, their monstrous form puts them well into "boss" territory. Despite its 6-point armour—making firearms practically useless—there are other weapons out there, so they are not completely unkillable. Their weather control abilities would also have plenty of narrative uses.

In a film...

The Spawn of the Winds could work well in a film. Rather than have them as a monster, however, I would be more inclined to have a character that didn't know they were one, and perhaps were never consciously aware that they'd been affecting the weather their whole life until a particularly bad storm hits at the moment they're getting worked up, or perhaps it starts snowing out of season when they're mourning the loss of a loved one. Their monstrous form may only appear in the climax... if at all.