Chain Assassination

• WOTC Official Reminder Textofficial rules
  Freerunning — Freerunning [cost] (You may cast this spell for its freerunning cost if you dealt combat damage to a player this turn with an Assassin or Commander.)

• WWizards of the Coast2024-07-05
  Official Ruling
  Once an Assassin creature or commander you control deals combat damage to a player in a turn, you can cast spells for their freerunning costs for the rest of that turn. It doesn’t matter what happens to that Assassin or commander after that.
• WWizards of the Coast2024-07-05
  Official Ruling
  If the target creature is a legal target but isn’t destroyed, most likely because it has indestructible, you’ll still draw a card if another creature died this turn.
• WWizards of the Coast2024-07-05
  Official Ruling
  If the target creature is an illegal target by the time Chain Assassination tries to resolve, it won’t resolve and none of its effects will happen. You won’t draw a card even if another creature has died this turn.

Rulings via Scryfall