The Monk class gains the deflect missiles ability at level 3:
Deflect Missiles
Starting at 3rd level, you can use your reaction to deflect or catch the missile when you are hit by a ranged weapon attack. When you do so, the damage you take from the attack is reduced by 1d10 + your Dexterity modifier + your monk level. If you reduce the damage to 0, you can catch the missile if it is small enough for you to hold in one hand and you have at least one hand free.
If you catch a missile in this way, you can spend 1 ki point to make a ranged attack with the weapon or piece of ammunition you just caught, as part of the same reaction. You make this attack with proficiency regardless of your weapon proficiencies, and the missile counts as a monk weapon for the attack, which has a normal range of 20 feet and a long range of 60 feet.
What happens when the damage from the attack includes damage types which the monk is immune or resistant to? At level 10, for example, the monk becomes immune to poison. So a poisoned arrow from an Assassin would only do piercing damage to the monk. If the monk was under the effect of the Blade Ward spell they would be resistant to the piercing damage from the arrow.
So if a level 10 monk under the influence of a Blade Ward spell was shot at by an assassin and the attack would succeed. What damage total is the monk trying to beat? The full damage roll of the piercing and poison damage? Half of the piercing damage alone?
