Insightful Maneuver 2014

1st-level divination

Casting time: 1 bonus action
Range: Touch
Components: V, S
Duration: instantaneous

With a flash of insight, you know how to take advantage of your foe’s vulnerabilities. Until the end of your turn, the target has vulnerability to one type of damage (your choice). Additionally, if the target has any other vulnerabilities, you learn them.

Source: Deep Magic for 5th Edition (Kobold Press) · License: OPEN GAME LICENSE Version 1.0a details