Simple concept 
If two units attack each other at the same time it counts as a parry and they either deal no damage or half damage only 1 attack can be parried at a time
If you UC a unit and attack a projectile the moment it hits them then it will parry the projectile and deal no damage (AI will not attempt this only UC)
Debuff attacks can be parried but not the debuffs themselves for instance if a toxic dead is parried then your UC will still get poisoned
Special abilities such as sword jump, jugger charge, fullminate, ect cannot parry and cannot be parried with the exception of Tactical roll and shadowrath which can parry
Stun and splash attacks (as well as the base damage from the attacks) cannot parry and cannot be parried except by shadowrath this keeps their parry ability unique and will still stun it wall also make ripriders and sicklewrath more consistent and not have rips be virtually immortal Marrowkia will also not be able to be parried except by shadow 
If the parry mechanic blocks 100% damage then after 2-3 consecutive parries damage is dealt anyway this counts for projectiles too
Bombers are also completely unaffected by parries as it could render them useless against an army mid fight 
As for if this should be UC specific idk cause in some cases this could make battles last allot longer but it just makes sense for every unit to parry. Either way it'll definitely add some skill into the game and give an edge against brain dead archer cowards as well as giving some other units a little more usability


