If you are unable to sacrifice 12 power worth of creatures, then you cannot choose that option; in that case, you must sacrifice Phyrexian Dreadnought.
15-07-2007
wotc
If the Dreadnought is no longer on the battlefield when the triggered ability resolves, you may still choose either option.
15-07-2007
wotc
Reverted to its original wording, this now has an "enters" triggered ability. During resolution of the triggered ability, you choose one option or the other.
04-10-2004
wotc
Phasing in does not trigger "enters" abilities, so you don't have to sacrifice again if it phases in.
04-10-2004
wotc
The sacrifices are simultaneous, not one at a time.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.