Aetherplasm

• WWizards of the Coast2006-02-01
  Official Ruling
  If you want, you can return Aetherplasm to your hand and not put anything onto the battlefield. The attacking creature will still be considered blocked, so it won't be able to deal combat damage to you unless it has trample or a similar ability.
• WWizards of the Coast2006-02-01
  Official Ruling
  Aetherplasm will return to your hand before any combat damage is assigned.
• WWizards of the Coast2006-02-01
  Official Ruling
  The creature you put onto the battlefield from your hand is blocking the attacking creature, even if the block couldn't legally be declared (for example, if that creature enters tapped, or it can't block, or the attacking creature has protection from it).
• WWizards of the Coast2006-02-01
  Official Ruling
  The creature you put onto the battlefield may be the Aetherplasm you just returned to your hand. This allows it to dodge abilities like Deathgazer's.
• WWizards of the Coast2024-01-12
  Official Ruling
  Although the creature that enters the battlefield is blocking, it was never declared as a blocking creature for the purposes of abilities that trigger whenever a creature blocks (such as Aetherplasm's ability, for example).

Rulings via Scryfall