Imagine a metal that can be molded into (almost) any form using magic, but once the process is done the material becomes absolutely immutable. It is not possible to physically alter its form. It can be reformed using magic, but the process is slow, difficult and energy-consuming.
It is supposed to be a heavy material, twice as heavy as your average steel. In its immutable state it is completely chemically inert. It does not expand when being heated to any temperature and does not get brittle when cooled. No impact can deform it, it will only be accelerated as a whole. $F=m\times a$
It has the limitation that the material needs to have a minimal thickness of about 1 milimeter to be perfectly immutable. It does not have to fulfill this requirement everywhere, but every position (including edges) needs to be directly connected and close to a portion that is sufficiently thick.
This means you can have thin (or even sharp) edges, but they need to be extending from sufficiently thick part and can't be long.
Would this material be superior to traditional materials for armour in a roughly medieval setting?
The classical problem of armour is weight. This would still be the case with this material as your entire armour could be a sheet of 1mm steel, which is just slightly thinner than medieval plate armours (1.5-3mm).
Another issue I see is heavy impacts. The reason modern cars are not as rigid as cars were 50 years ago is that they can absorb a significant amount if the impact with the crumple zone and also that the acceleration is not nearly as strong on the person.
With the old rigid designs you are bound to die in a high velocity accident, even if your car wasn't deformed at all.
Of course, the forces that tend to work on soldiers are usually significantly lower than on drivers in a high velocity impact, but I am not certain whether the problem is irrelevant as I see the distribution of the impact on the body posing a problem. It might not be worse than it was with other medieval armour, though.
Answers should elaborate on whether adaptions or changes to traditional armour designs would be required to make the armour profit from the unique material properties and offer advantages over the traditional armour .
This question is part of a series regarding weapon and armour design using fictional materials with unique properties