Read rules for Knight of the Mists
Flanking (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)
When this creature enters, you may pay 
. If you don't, destroy target Knight and it can't be regenerated.
. If you don't, destroy target Knight and it can't be regenerated.2/2
: Choose a creature type other than Wall. Target creature becomes that type until end of turn.