Usually, the image is called stem wrench. The stem turning nut may be buried in the wall close to the valve body, for 2 or 3 handle valves. Not a plumber and saw just a few brands, but that wrench may be what you need. Or as huesmann says, what you see there. It helps to know the brand. Sometimes brand is on the handle you removed, sometimes not.
To use the wrench, it looks like you may have to cut open more space around that stem. Need the space to see better and to insert the wrench. A grinder (very carefully), oscillating saw (can jump and skip), one of those very small grinder, and the escuchon with foam seal or caulk cover later.