Seadra

You may sacrifice a creature named Horsea
rather than pay this spells mana cost.
As long as the defending player this turn controls
an Island, Seadra has "{t}: Blue creatures you
control gain hexproof and flanking until end of
turn. (Whenever this creature becomes blocked
by a creature without flanking, the blocking creature
gets -1/-1 until end of turn.)"
{u}, {t}: Seadra deals 6 damage to target creature. {u}{u}{u}, {t}: Target player's creatures get -1/-1
until