You can hard cast an Eldrazi
Usually the correct choice is to never take the offer, though for a table there has to be some implicit trust since to be the only opponent to take an offer gains that player an advantage over everyone but the offering player
The wildcard wheels on the top right advance when you open packs, and cycle through mythic as well
However, if no creatures have died under your control so far this turn as your end step begins, the ability won't trigger at all
When you play Chivalrous Chevalier, you have to either bounce a creature back to your hand, or you have to compliment an opponent, which is just nice
In many ways its best value is being able to flash in end of turn and be two devotion higher than your opponent was expecting