Necra

Beware these creatures that roam the inside of the Sea Mountains. They seemingly exist without a purpose, simply slowly walking the inside of the many caves. While they don't usually attack many of the monsters that live inside, they will without question attack other humanoids (monster or not).
The history of the Sea Mountains are not that well known. Even trying to see what old scrolls and books might say about them don't reveal much. As a result, these creatures, while humanoid-esque, have a very low chance of coming from actual people. They might be monsters that evolved to look humanoid, but for what purpose no one truly knows. It's speculated that there must a connection somehow since they seem to hate humanoids so much. It also seems like they don't truly distinguish proper humanoids. A vague shape of a humanoid is enough for them to attack whatever has said shape. They'll often be in small groups, roaming together, which is very strange that they don't attack each other. Which means that is some kind of connection between all of them that we don't know about yet. As they finally kill their targets, they burn the bodies and carry around the ashes in their cage they carry. While it may have bars that hold it together, the ash doesn't seep out of them. It seems the ash stays inside the cage using magic. Once a Necra dies, any ash that is stored inside the cage finally seeps through the metal.
Interestingly enough, the sword that they carry isn't a sword at all. While it may resemble one and feel like one, it's actually a part of the Necra's body itself. Very likely a type made of sharp and incredibly dense bone that protrudes from their one arm. Be cautious as this "sword" may be heated up to dangerous proportions, searing any creatures that might be hit by it. In fact it's hot enough to instantly cautorize any wounds the sharp blade may make. Though also be weary of the flame inside its head. Once it starts growing bigger, it makes the Necra more powerful and dangerous.