Kardur, Doomscourge

• WWizards of the Coast2021-02-05
  Official Ruling
  Kardur's last ability will trigger no matter who controlled the attacking creature and no matter who it was attacking.
• WWizards of the Coast2021-02-05
  Official Ruling
  If a creature an opponent controls doesn't qualify for any of the above exceptions and can attack, it must attack a player other than you if able. If the creature can't attack any of those players, or if there are no such players, it must attack you or an opposing planeswalker.
• WWizards of the Coast2021-02-05
  Official Ruling
  Kardur's first ability affects all creatures your opponents control, including any that enter the battlefield after the ability resolves.
• WWizards of the Coast2021-02-05
  Official Ruling
  If a creature affected by Kardur's ability could legally attack multiple players and/or planeswalkers, its controller chooses who it attacks.
• WWizards of the Coast2021-02-05
  Official Ruling
  If a creature an opponent controls can't attack for some reason (such as being tapped or having come under its controller's control that turn), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost, so the creature doesn't have to attack in that case either.

Rulings via Scryfall