The were in the middle of escaping the cave and i think lucas was supposed to use his magic to help make a way but instead he attacked them making them a sacrifice to distract the guradian for him to escape alone.
I think that when they were trying to escape or trying to beat the elder guardian Lucas had used his magic to make Arthur and the rest of the members bait so he could escape.
Can someone explain what happened with lucas I get he betrayed them but I forgot What he did and I cant figute it out from rereading lol