monsters – 5e creatures that ignore armor?

I’m trying to homebrew a nasty hornet swarm with some added attacks to make it more dangerous. One of the ideas that I had was to make the hornets crawl under the armor/clothes when attacking, thus ignoring the armor/shield bonus. I was wondering if there are already mechanics for something like this in the game or monsters that do something similar. I could not find anything during my search, but my book access/knowledge is not absolute unfortunately.

If there are no rules for it, I would probably give them an ability like:

Attack from within By spending 5 feet of its movement, the swarm can crawl in between the armor of the target, attacking them directly. Use a target AC of 10 + Dexterity modifier for any attacks made in this state unless the target is wearing protective clothing, has unarmored defense or similar. This effect ends by the end of the swarm’s turn when it crawls back out.