wotc
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.
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.
If the Dreadnought is no longer on the battlefield when the triggered ability resolves, you may still choose either option.
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Pete Venters
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
Phasing in does not trigger "enters" abilities, so you don't have to sacrifice again if it phases in.
The sacrifices are simultaneous, not one at a time.