If a creature can’t legally block Breaker of Armies but could block another attacking creature, it may do so. Likewise, if a creature the defending player controls can’t block Breaker of Armies unless its controller pays a cost, its controller may decline to pay that cost and block a different attacking creature.

If, during its controller’s declare blockers step, a creature the defending player controls is tapped or is affected by a spell or ability that says it can’t block, then it doesn’t block. If there’s a cost associated with having it block, its controller isn’t forced to pay that cost. If they don’t, the creature doesn’t have to block.

If more than one Breaker of Armies is attacking, the controller of each creature that could block them chooses which one that creature blocks. In this case, creatures that can block multiple creatures must block as many attacking Breaker of Armies as possible.