Xenoform

Target creature phases out with three time
counters on it. At the beginning of each upkeep,
remove a time counter. When the last time
counter is removed, it phases back in, and you put
+1/+1 counters on it equal to the difference left
before it reaches 4 power and toughness,
otherwise put a flying and trample counter on it,
and two +1/+1 counters. That creature becomes
a merfolk horror in addition to its types.