From Destinypedia, the Destiny wiki
Phalanx are a type of Cabal military rank.
They are similar to Legionnaires and carry big tactical shields, acting as a mobile defensive front, holding their ground or pushing forward at will; they have no jump-jets. Besides blocking attacks, Phalanx shields are dangerous in melee combat. A single hit can send Guardians flying, which will cause damage if they crash into a solid object. This is strong enough to kill Guardians on higher difficulties. In Destiny 2 their shields have a deactivation point at the center, which can be shot for minimal damage but will retract the shield and briefly stun the Phalanx.
The Phalanx's shield they carry covers a big portion of their body and is immune to all incoming damage, and their wielder will occasionally plant it down to fire their Slug Rifle in rapid succession. Otherwise, the Phalanx will slowly corner a Guardian until they get into melee range, where they will either quickly swing their shield at them or lunge towards their target and pound them with the shield, which can outright kill the player in one hit or send them flying if they happen to survive. To prevent them from getting close, it's advised to either shoot their exposed arm to stun them or kill them outright with splash damage from grenades, Rocket Launchers or Interceptor mortar shells. 
In Destiny 2, Red Legion Phalanxes are equipped with Energy Shields, which they can expand to protect multiple Cabal soldiers. They do not impede movement when expanded and can be walked through. Shooting the glowing icon in the center of the shield or using a Suppressor Grenade will deactivate the shield and stun the Phalanx for a few seconds. Unlike Legionnaires they tend to use Slug Rifles rather than Cabal Slug Shotguns. Red Legion Phalanxes are now equipped with jump jets like other Cabal and are thus capable of limited flight.
List of appearances