Lurking Arynx

• WWizards of the Coast2015-02-25
  Official Ruling
  If you control a creature with power less than 0, use its actual power when calculating the total power of creatures you control. For example, if you control three creatures with powers 4, 5, and -2, the total power of creatures you control is 7.
• WWizards of the Coast2015-02-25
  Official Ruling
  Some formidable abilities are activated abilities that require creatures you control to have total power 8 or greater. Once you activate these abilities, it doesn’t matter what happens to the total power of creatures you control.
• WWizards of the Coast2015-02-25
  Official Ruling
  Activating Lurking Arynx’s ability doesn’t force you to attack with it that turn. If it doesn’t attack, the creatures affected by its ability can block other attacking creatures or not block at all.
• WWizards of the Coast2015-02-25
  Official Ruling
  Other formidable abilities are triggered abilities with an “intervening ‘if’” clause. Such abilities check the total power of creatures you control twice: once at the appropriate time to see if the ability will trigger, and again as the ability tries to resolve. If, at that time, the total power of creatures you control is no longer 8 or greater, the ability will have no effect.
• WWizards of the Coast2015-02-25
  Official Ruling
  You may activate Lurking Arynx’s ability multiple times in a turn to force multiple creatures to block it if able.
• WWizards of the Coast2015-02-25
  Official Ruling
  If a creature affected by Lurking Arynx’s ability can’t legally block it (perhaps because Lurking Arynx has gained flying), that creature can block other attacking creatures or not block at all.
• WWizards of the Coast2015-02-25
  Official Ruling
  If, during the declare blockers step, a creature is tapped or is affected by a spell or ability that says it can’t block, it doesn’t block. If there’s a cost associated with having a creature block, its controller isn’t forced to pay that cost, so it doesn’t have to block in that case either.

Rulings via Scryfall