From Destinypedia, the Destiny wiki
Common Legionary tactics focus on the bounding advance: while some units attack the target, others attempt to close the gap between them or find new firing positions.  They are supported by Centurions and Phalanxes,  and their heavy armor and weaponry is enough to overcome even the bravest of Guardians.
Legionaries are capable of holding thousands of Slug rounds. 
It appears that the Legionaries have a medical system in their suits which acts as a pressurized tourniquet that seals wounds or prevent blood loss, and the emergency medic release is located in their helmet that releases the pressure seal and the oil fluid seeps out. 
Legionaries are the most common troops found in the Cabal army and are primarily armed with Slug Rifles or, in rare cases, Projection Rifles. In battle, they use large numbers and overwhelming firepower to overrun and wipe out their enemies. Due to their heavy armor, They are the largest and most durable basic infantry from all of the enemy races. While this armor makes them appear slow and cumbersome, they are actually able to move around the battlefield thanks to the jump jets built into their armor. Because they often fight in groups and in close proximity to each other, they can be quickly felled by area-of-effect weapons and attacks, such as Rocket Launchers and grenades.
In Destiny 2, Legionaries remain largely the same, except they now have the ability to deploy blades from their free wrist to make melee attacks. These blades can also be fired at the player, and will explode like a fragmentation grenade a few seconds after making contact with any surface. They typically wield Cabal Slug Shotguns, though some Legionnaires continue to wield Slug Rifles. 
List of appearances