Page 20 of the rule book.
"Unless the card says otherwise, conjurations affect the entire game board, regardless of the range."
Thanks guys.
I was pretty sure that the Obelisk effects all Non-mage creatures in the arena, actually - but it seemed to be a little too powerful. It hampers the beastmaster, doesn't it?
Only if the Beastmaster is playing a swarm strategy, which is not his only option, but normally yes it helps against the Beastmaster.
Also remember that the Obelisk will affect the caster's creatures as well, not just his opponent's.