Siege of Storm's End

When Siege of Storm's End enters the
battlefield, choose Attackers or Defenders.
• Attackers — At the beginning of your upkeep,
put a +1/+1 counter on up to two target
creatures you control, they gain horsemanship
until end of turn.
• Defenders — Whenever a creature attacks you,
it gets -1/-1 and loses horsemanship until end
of turn.