Rotten Kelpie

When Rotten Kelpie blocks or becomes blocked
by a creature, that creature's controller may draw
a card. If they do, you may pay {b}. If you do,
you gain 5 life and they lose 5 life.
{0}: Prevent all combat damage that would be
dealt to and dealt by Rotten Kelpie until end of
turn. Any opponent may activate this ability.