Belzoma, Shaper Potentate

When Belzoma, Shaper Potentate enters play,
if you cast it, and at the beginning of your
pre-combat main phase, each opponent in turn
reveals the top card of their library. They may
put that card into their hand. If they do, you
add {g}{u}. Otherwise they put it on the bottom
of their library and you gain 2 life.