Oracle Text

As Conspiracy enters the battlefield, choose a creature type.

Creature cards you own that aren't on the battlefield, creature spells you control, and creatures you control are the chosen type.

Card Rulings

10/4/2004 It does not replace any use of creature types in card text.
10/4/2004 This can grant a creature type to animated lands and artifacts that would otherwise have no creature type.
8/1/2005 "Legend" is no longer a creature type, and may not be chosen.
8/1/2005 If you choose Wall, then your creatures can still attack because creature types don't confer abilities such as defender (any more than, say, choosing creature type Bird would confer flying).
9/20/2013 Affected spells, cards, and creatures lose all of their other creature types.