Artifact piece of armor (the exact slot is unspecified) which, when worn, ensures you have at minimum 10 points of AC. So for instance, if the artifact’s base type grants 1 AC, and it’s +0 and you wear only it, your AC is 0. If you put on 9 AC more worth of armor, your AC will still be 0. But if you subsequently put on 1 additional AC worth of armor so you have 11 total points, your AC will become -1 and the artifact will have no further effect.

This is intended explicitly as an early-game artifact.