Is it from the verb 居る? If it's so, why is not it ております?
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In very short: it is the humble form of いる.
To give a typical example, おる is mostly used when talking about your own actions and trying to be humble (for example when talking to a superior such as your boss at work, to a customer, etc). Find something about this topic here or check the wiki about Japanese honorific speech.
In your specific example, I believe the sentence continues with something you haven't written here. In fact, おる is in the "pre-masu" form which is often used in written language to connect to the next sentence. Refer to this answer for more details on this.
EDIT: So I made some research and I think your full sentence is in fact:
Is it correct?
So I think we can translate this as (until the comma): the damage due to the (less literally, left by) the mysterious person (monster? Not sure what 怪人 is depending on the context) is expanding at an unprecedented scale.