Official Mechanic:Legion

"Seek Mate... ...Legion!"

Legion (Japanese: Region) is a keyword ability that allows two cards on the Vanguard Circle, in which both units on it are Vanguards as "Legion", and join forces during attack.

Mechanics
"Legion" is an Activated ability that can be used if the unit itself has never been in Legion, and both players' Vanguards are Grade 3 or greater.

After the conditions are fulfilled, the player can perform Legion by following these steps.
 * 1) Declare the use of "Legion" ability; the process is called "Seek Mate" (Japanese: シークメイト Shīku Meito).
 * 2) Pay the cost: Shuffling 4 cards from their drop zone into the Deck.
 * 3) Search their deck for up to one card listed next to the ability.
 * 4) If found, place the card on their Vanguard Circle.

The placed card, now known as "Legion Mate" remains there for the rest of the game.