Phyrexian Dreadnought

Oct 4, 2004 · Phasing in does not trigger "enters" abilities, so you don't have to sacrifice again if it phases in.

Oct 4, 2004 · The sacrifices are simultaneous, not one at a time.

Jul 15, 2007 · If the Dreadnought is no longer on the battlefield when the triggered ability resolves, you may still choose either option.

Jul 15, 2007 · 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.

Apr 15, 2013 · 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.