1 Feb 2007
In the Two-Headed Giant format, you still only have to pay once per creature.
1 Oct 2008
This has the supertype world. When a world permanent enters, any world permanents that were already on the battlefield are put into their owners' graveyards. This is a state-based action called the "world rule." The new world permanent stays on the battlefield. If two world permanents enter at the same time, they're both put into their owners' graveyards.
1 Feb 2014
Unless some effect explicitly says otherwise, a creature that can't attack you can still attack a planeswalker you control.