If both targets are illegal, the spell won't resolve at all
Slayers Stronghold ($2) This is in a lot more decks that you think, and aside from the SL version thats all in black, one of the only special versions out there
Blizzaga mostly just offers a way to trigger Tellah, Great Sage or Ultros, Obnoxious Octopus
Regardless, protection is still a powerful and beloved keyword ability with some fantastic cards
If instead the creature you control is an illegal target but the creature you don't control is still a legal target, nothing will happen when Rabid Gnaw resolves
Cerberus Rarity: Rare TYPE: Enchantment Creature SUBTYPE: Saga Dog CMC: 4 Mana cost: USD: $0.33 EUR: 0.24 TIX: 0.02 EDHREC RANK: 7877 (3/3) (As this Saga enters and after your draw step, add a lore counter