It isn't in 5.0 it seems since germany got it a bit before 5.0 same for skill expirement except they only have gotten 2 ninjas for it . And regarding how it works you where about right it does indeed use certain ninjas to have the bond kill activate. But you also need to have the cards leveled up to around level 26 before the bond mystery appears. Since you can only put 4 cards on 1 ninja that would mean you have to have like 7 + 7 + 5 + 5