Golem Sapper
This octagonal device is roughly fist sized and weighs 1/2 pound. As an action, you can place the device on a construct within your reach. When you do, the creature must make a DC 18 Constitution saving throw. On a failed save, the creature takes 2d10 necrotic damage and is stunned until the start of your next turn, at which point it can repeat the saving throw. Each time the creature makes this saving throw after the first, the DC is reduced by 2 (to a minimum DC of 10). On a successful save, the device falls from the creature and can't be used again for 1 hour.
The device's saving throw DC remains reduced after being used. The DC increases by 2 for every 24 hours it isn't in use, up to a maximum DC of 18.
The Ravenheart strode through the catacombs beneath the manor, carefully adjusting the unfamiliar red coat and turning the clawed device over and over in his hand. Even before he turned the corner he heard the regular ring of the sentry - an untiring clockwork guardian for the resting place of one Cornelia Shutter, brilliant inventor and original owner of the home above which had, of late, hosted a succession of proud nobles who had failed to get past the creation's glowing gaze. The thief, however, walked into its sight with confidence - a tool necessary to his art. He had uncovered two weaknesses: first, Shutter had assumed that any in her distinct uniform were her allies; second, the sentry would only note weapons that, to any, would look like weapons, and Shutter could never have predicted his gift for vulnerabilities. He carefully unlocked the crypt's equally complex door even as the final, sparking rattle of the sentry gave way to silence. The geared blade resting at its heart was soon gone, replaced with a now-iconic note.