(Image taken from Nerp's Deviantart)
If the Philippines has an iconic mythical monster, it would be the Aswang, if only because it's been overused in many local films. There are several stories about Aswangs--some that are even conflicting--but they can be boiled down to one of two variants. The first, and most popular, is that of the Manananggal (the root word is tanggal which means to separate). Here, the Aswang is a comely maiden by day but by night, transforms into a monstrous form: her upper body separates from her lower body and she grows wings, making her a mobile predator. Aside from being a glorified Angel of Death minus the legs, the Aswang has the ability to extend its tongue. This is usually utilized to feed on unborn babies of pregnant women (usually when they are sleeping). I could easily imagine that Aswangs were born out of a need to explain miscarriages. However, what makes the Aswang truly unique is its weakness. There are two ways to defeat an Aswang. One is that they die when they touch sunlight while in their monstrous form. That is usually achieved by the second method, which is to prevent Aswangs from returning to their lower half. Now there have been several ways to accomplish the latter. The most common way is to sprinkle salt or ashes or spices on the vulnerable lower half of the Aswang. The presence of those materials prevent the Aswang from returning to their body. The other way is to hide the body or in the case of multiple Aswangs, confuse them. Apparently, the Aswang's lower half is immobile. When you have several Aswangs transforming at the same time, they memorize the location of their bodies. If you switch those bodies around, apparently Aswangs also suffer from an inability to tell which is their corresponding half and so are unable to return to their human forms.
Much like the Tiyanak or the Werewolf, the terror of the Aswang is its ability to disguise itself as a normal human. Your story could easily revolve around a mystery, figuring out who the assassin is and how they are managing to slay targets that are in otherwise difficult-to-reach locations (not a problem thanks to the Aswang's flight). If you want to turn the tables around, the Aswang could be the victim as a rapist or pervert is preying on the Aswang's bottom half while it is out on the hunt. The Aswang could also benefit from some cosmetic changes. For example, the 10th Plague in the Bible could have easily been caused by Aswangs. At midnight, the first-born children are hunted by Aswangs who bear angel-like wings. Better yet, the first-born child can be altered so that Aswangs are more true to their mythology and instead of a plague of first-born deaths, it is a plague of miscarriages. In science fiction, the Aswang can be an archetype for a certain device. The upper torso is a mobile, killing machine that is virtually invincible but at the end of the day, needs to return to its bottom half to recharge its power supply. While recharging, the sci-fi Aswang can scout and explore but has no access to its weaponry or its invulnerabilities.
Variations: The Manananggal Aswang attributes its transformation to one of two sources. In one story, the women must rub a special oil on their bodies in order to transform. In another, they are possessed by an evil spirit of sorts and they can be "cured" by causing them to vomit a black, bird-like creature. The other Aswang variant which I didn't mention above is that Aswangs aren't women who transform into horrifying monsters that rather Aswangs are witches born with the ability to curse and cause misfortune on unwary people they meet. One could easily combine the two versions, with Aswangs having magical powers while in human form but lose it when it's evening.