Alicia casts Roar of Challenge on her Runeclaw Bear and attacks Nash with it and Hill Giant. Nash controls a Qal Sisma Behemoth and two untapped lands.

Nash must obeys as many requirements as possible in his declaration of blockers, but he is not forced to pay the Behemoth's cost. So, “no blocks” is a legal choice, which obeys zero requirements.

Is he allowed to pay and block the Hill Giant?

No, they could not block the Hill Giant. We have to fulfill as many requirements as we can without violating any restrictions. You can choose not to pay 2, and then the Behemoth can't block (and we are not forced to pay 2 to maximize requirements, due to 509.1c). If you choose to block with the Behemoth, then it has to block the Roared Runeclaw Bear, since that will maximize the blocking requirements. Blocking the Giant leave that requirement unfulfilled when you could fulfill it, so it is not a legal block.

In short, you have two choices with the Behemoth: you can choose not to block with the Behemoth (since you are not required to pay 2), or the Behemoth can block the Bear (by paying 2 to enable it to block).

Nathan Long
Wizards.Com Boards NetRep