Ahistorical source can be deemed unreliable if the author appears to unfairly favor a particular side or viewpoint. in this case, the source is called

The correct answer is Bias.

A trustworthy source is one that passes factual knowledge without much bias (where Bias means holding an unfair or fallacious opinion). In simple terms, Bias is giving preference for one thing over another. A source is fully reliable if it does not show bias. History is a subject where people reveal their opinions, hence, we have to be very cautious while going through history.  

However, Bias in the source does not automatically make a source untrue or incorrect.  Recognizing which side the source favors only allow us to highlight the gaps in the information.