If the light coming out of the amulet is well collimated (i.e. it can form a narrow beam that does not disperse - like a laser) and the light is emitted continuously rather than in pulses, then what it does will depending on the aperture (i.e. the cross sectional area) of the amulet.
Assuming a diameter of 2-3 cm (wristwatch or locket sized), an intensity of tens to hundreds of watts could cause temporary/permanent blindness if pointed at someone's eyes, and kilowatts would be enough to set light to a flammable target. 104 of W would cut through stuff. If the emission moves up to megawatts then the surface of the target will heat up and ablate so rapidly that it will effectively act like an explosion - the target might be thrown backwards but this is due to evaporation at the super-heated surface, not radiation pressure. Somewhere around 1012-1014W, the air in the path of the beam would almost instantaneously ionize and absorb a significant fraction of the beam energy. This would be bad for the holder of the amulet as it means that the air directly in front of them would glow more brightly than the surface of the sun - the amulet holder would get very bad sun-burn.
At vastly higher energy the beam could convert materials to plasma, break up atomic nuclei and eventually convert all matter to a quark-soup like that found in the first few nanoseconds of the big-bang. Radiation pressure would now be evident - but only if you used the amulet in a vacuum as the reaction with any nearby matter would overwhelm the effect otherwise. At stupidly higher energy, you might reach the Plank limit beyond which physicists can only guess - but more likely the energy density would be so great that local space along the beam would instantly collapse into a black hole.
If the amulet fires out a narrow beam - 0.1 mm diameter perhaps, then all the power requirements drop by around 4 orders of magnitude. Probably harder to blind an enemy then but it would still be effective for cutting/drilling through things at a distance. If it was pulsed, then the relative danger to the user may be reduced.
If the amulet fired out a dispersed wide angle multi-frequency beam like a hand-held torch, then bad things would happen to the wielder (the sun-burn and explosions resulting from heating of the air immediately in front of the amulet) before it would do much harm to any target more than a few tens of metres away.
If the light is not coherent, all the things described for the coherent beam would still happen, but at higher energy required for an effect on a target, and a lower energy required for the 'effects' on the wielder.