Obelisk of false promise

At the beginning of your turn, each mutant
creature gets -1/-1
{t}{g}{b}{1}: Sacrifice two non-mutant creatures,
create an X/Y mutant creature token where X is
the combined power of the two creatures and Y
is the combined toughness. The mutant creature
token gets all the abilities of the two sacrificed creatures (Flying, first strike, etc). Or gets all activated abilities of the two sacrificed creatures.