A Sword Brethren or Sword Brother, are the Black Templars equivalent to the First Company of other Chapters. Black Templar veterans are elevated to this rank and placed in separate squads where they will fight as elites, train to use Terminator Armour (they are the only Black Templars allowed to use the armour) or form a Marshal's bodyguard.
During Crusades, a Marshal can also appoint a Castellan to lead one of the Fighting Companies. This Castellan is picked from the Sword Brethren as is the case when a Castellan or Marshal is killed, the new one will be picked from the Sword Brethren.
Despite the fluff of them all wearing Terminator armour, in terms of the minis, none of them actually wear it. Why that's the case, we have no clue.
In 6th Edition, these guys are more expensive Initiates (that don't score). They have the same options as initiates (1 special weapon, 1 heavy weapon Or special close combat weapon). Plus, can have "terminator Honors" to each gain an additional attack (but it's crazy expensive) and can have as many storm shields (FAQ'd to 3++ all the time, but got more expensive) as your little heart desires. I suggest you give them a skip.
|Forces of the Black Templars|
|Command:||High Marshal - Marshal - Castellan - Emperor's Champion |
Brother-Captain - Chaplain - Techmarine - Apothecary
|Troops:||Sword Brethren - Terminator Squad |
Black Templar Neophyte - Initiate
|Vehicles:||Bike Squad - Rhino - Razorback - Land Speeder |
Predator - Vindicator - Whirlwind - Stalker - Hunter
Land Raider (Land Raider Redeemer - Land Raider Crusader)
|Flyers:||Thunderhawk - Stormraven - Stormtalon |
Stormhawk - Storm Eagle
|Spacecraft:||Drop Pod - Boarding Torpedo |
Space Marine Landing Craft
|Allies:||Sisters of Battle - Space Marines - Primaris Marines|