Eradicator Valkyrie

• WOTC Official Reminder Textofficial rules
  Lifelink — Damage dealt by this creature also causes you to gain that much life.
• WOTC Official Reminder Textofficial rules
  Hexproof — This permanent can't be the target of spells or abilities your opponents control.
• WOTC Official Reminder Textofficial rules
  Boast — Activate only if this creature attacked this turn and only once each turn.

• WWizards of the Coast2021-02-05
  Official Ruling
  Eradicator Valkyrie can't be the target of abilities whose source is a planeswalker an opponent controls.
• WWizards of the Coast2021-02-05
  Official Ruling
  As the boast ability resolves, first the next opponent in turn order chooses a creature or planeswalker they control. Then each other opponent in turn order chooses a creature or planeswalker they control, knowing the choices made before their choice. Then all chosen creatures and planeswalkers are sacrificed at the same time.
• WWizards of the Coast2021-02-05
  Official Ruling
  If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.
• WWizards of the Coast2021-02-05
  Official Ruling
  If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.
• WWizards of the Coast2021-02-05
  Official Ruling
  If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.
• WWizards of the Coast2021-02-05
  Official Ruling
  A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.

Rulings via Scryfall