Even worse, and something the other answers didn't mention, is that it's impossible to shape diamond into the items you're looking for. The crystal lattice simply won't allow for it.
So the suit of armour AND the weapon are theoretical impossibilities, at least if you make the assumption they're each made out of a single crystal.
If they're made of many crystals, bonded together in some way, things get worse as now you have a multitude of bonds between crystals that are weak spots in your armour and failure points in your weapon quite apart from the brittleness of the diamond itself.
And then there are the required joints between the armour pieces, the gaps between them, that allow the wearer to actually move in the armour. Medieval fighting techniques for dealing with people wearing plate armour (which this would be a ludicrous variant on) often included ways to dislodge armour plates by driving items under them and prying them loose, or sticking a something pointy in between plates to cause wounds under them. Both techiques would work with a suit of armour made out of diamond as well.

Of course in a fantasy universe maybe something else is meant by the term "diamond", maybe some white semi-translucent metalic compound that is extremely hard and expensive.
In that case it might be forged into swords and armour plating in a more normal fashion (rather than gluing bits together) but you'd still have the gaps between the plates that can be used to cause wounds or get plates off and then wound the wearer.

Even worse, and something the other answers didn't mention, is that it's impossible to shape diamond into the items you're looking for. The crystal lattice simply won't allow for it.
So the suit of armour AND the weapon are theoretical impossibilities, at least if you make the assumption they're each made out of a single crystal.
If they're made of many crystals, bonded together in some way, things get worse as now you have a multitude of bonds between crystals that are weak spots in your armour and failure points in your weapon quite apart from the brittleness of the diamond itself.
And then there are the required joints between the armour pieces, the gaps between them, that allow the wearer to actually move in the armour. Medieval fighting techniques for dealing with people wearing plate armour (which this would be a ludicrous variant on) often included ways to dislodge armour plates by driving items under them and prying them loose, or sticking a something pointy in between plates to cause wounds under them. Both techniques would work with a suit of armour made out of diamond as well.

Of course in a fantasy universe maybe something else is meant by the term "diamond", maybe some white semi-translucent metallic compound that is extremely hard and expensive.
In that case it might be forged into swords and armour plating in a more normal fashion (rather than gluing bits together) but you'd still have the gaps between the plates that can be used to cause wounds or get plates off and then wound the wearer.