Mercury, the Waterwyrm

Mercury, The Waterwyrm can't be blocked.
Shapeshift {1}{u}{u} ({1}{u}{u} Exile an attacking creature you control until Mercury,
the Waterwyrm leaves the battlefield, put this
card onto the battlefield tapped and attacking.).
If Mercury, the Waterwyrm entered the
battlefield tapped, tap target creature opponent controls. It does not untap during its controller's
untap step.
"One good turn deserves another."