Severed Strands

• WWizards of the Coast2018-10-05
  Official Ruling
  If the target creature is a legal target but can't be destroyed, most likely because it has indestructible, you still gain life.
• WWizards of the Coast2018-10-05
  Official Ruling
  You must sacrifice exactly one creature to cast this spell; you can't cast it without sacrificing a creature, and you can't sacrifice additional creatures.
• WWizards of the Coast2018-10-05
  Official Ruling
  The amount of life gained is equal to the sacrificed creature's toughness as it last existed on the battlefield.
• WWizards of the Coast2018-10-05
  Official Ruling
  If the target creature is an illegal target by the time Severed Strands tries to resolve, the spell doesn't resolve. You don't gain life.

Rulings via Scryfall