If an opponent has a Silent Arbiter, and I have a Territorial Hellkite & a second creature, can I swing with the other creature; or do I have to swing with the Hellkite?
Hellkite must attack, and the other cannot. (unless the random opponent somehow cannot be attacked by Hellkite)
508.1d (...) If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. (...) Example: A player controls two creatures: one that “attacks if able” and one with no abilities. An effect states “No more than one creature can attack each turn.” The only legal attack is for just the creature that “attacks if able” to attack. It’s illegal to attack with the other creature, attack with both, or attack with neither.
Example: A player controls two creatures: one that “attacks if able” and one with no abilities. An effect states “No more than one creature can attack each turn.” The only legal attack is for just the creature that “attacks if able” to attack. It’s illegal to attack with the other creature, attack with both, or attack with neither.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules