For all who watched S4, you would know about Legion. Legion is a new mechanic in Vanguard, released by Bushiroad in BT16.
What exactly is Legion?
It is an ACT ability to return 4 cards from your drop zone to your deck, search for a specified "mate" and perform Legion. The Legion Mate is usually a Grade 2, placed on the left of the Grade 3 unit.
How do I Legion?
When performing Legion, most people would send back 3-4 triggers or some key cards, depending on the contents of their drop zone. People generally want to perform Legion early for high numbers pressure. To do this, it is important to have 1-2 Quintet Walls in a deck consisting of Legion units. The QWs will help to build the drop zone quickly, allowing you to perform Legion sooner. However, do note that if you defend early-game, you may not have enough defenses for mid-late game. So do be careful on how you Legion :D
Examples of Legion units:
Royal Paladin: Seeker, Thing Saver Dragon + Blaster Blade Seeker
Shadow Paladin: Revenger, Phantom Blaster "Abyss" + Revenger, Blaster Dark "Abyss"
Genesis: Cosmic Regalia, CEO Yggdrasil + Regalia of Fate, Norn
Narukami: Brawler, Big Bang Knuckle Buster + Brawler, Big Bang Slash Buster
DP: Metalborg, Sin Buster + Metalborg, Ur Buster
DI: Psychier of Ash, Hadar + Psychier of Dust, Izaya(Monthly Bushiroad promo)
NN: Penoy Musketeer, Martina + Peony Musketeer, Thule
GP: Bluish Flame Liberator, Percival + Liberator of Oath, Aglovale
LJ: Star Vader, Garnet Star Dragon + Companion Star Vader, Photon
Spike Brothers: Bloody Ogre + Frozen Ogre
OTT: Battle Sister, Jelly + Battle Sister, Pudding(Monthly Bushiroad promo)
This is just a list of Legion cards that have already been released by Bushiroad. If your favourite clan doesn't have a Legion unit yet, fret not, because more Legion cards are coming your way in the upcoming boosters.
Remember, Vanguard is all about havig fun, so have fun, and Seek the Mate!! :D
For more information on Legion, do read
http://cardfight.wikia.com/wiki/Legion. They have an extensive article written on Legion here, for anyone who is still unsure of the mechanics :)