Which word denotes a destructive or harmful quality?

The word that denotes a destructive or harmful quality is "baneful." This term specifically describes something that causes harm or is destructive in nature. It is often associated with negative effects that can lead to suffering, ruin, or death, both in a literal and metaphorical sense. For example, one might refer to a substance that is toxic as baneful, as it has the capacity to cause significant harm to health or the environment.

In contrast, the other terms have different connotations. Bombast refers to language that is pompous or inflated, antithetical denotes something that is directly opposed or contrasted to another idea, and aspersion signifies a derogatory remark or criticism rather than implying direct harm or destruction. Therefore, "baneful" is the most fitting term in this context due to its clear association with harmful effects.