The noun bane refers to anything that is a cause of harm, ruin, or death. But we often use it for things that aren't that bad, just feel like it. You might say mosquitoes are the bane of your existence.
The source of this word is Middle and Old English bana, meaning "destroyer, murderer." The now obsolete meaning of "deadly poison" is seen in the names of poisonous plants such as wolfsbane and henbane. Although "bane of my existence" is a commonly heard phrase, there's something deliciously archaic about the word bane. It conjures up villages preyed upon by dragons, or witches adding one bane or another to a steaming kettle.