Lochmere Serpent

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2019-10-04
  Official Ruling
  You can't activate Lochmere Serpent's last ability without targeting five cards in a single opponent's graveyard.
• WWizards of the Coast2019-10-04
  Official Ruling
  An Island is a land with the subtype Island, not any land that could produce blue mana. For example, Mystic Sanctuary is an Island, but Castle Vantress isn't. Similarly, a Swamp is a land with the subtype Swamp, not any land that could produce black mana.
• WWizards of the Coast2019-10-04
  Official Ruling
  If up to four of the target cards are illegal targets by the time Lochmere Serpent's last ability resolves, the remaining legal targets are exiled and Lochmere Serpent is returned to your hand.
• WWizards of the Coast2019-10-04
  Official Ruling
  If each target card is an illegal target by the time Lochmere Serpent's last ability resolves, the entire ability won't resolve. Lochmere Serpent won't be returned.
• WWizards of the Coast2019-10-04
  Official Ruling
  Activating Lochmere Serpent's first activated ability after it has become blocked won't cause it to become unblocked.

Rulings via Scryfall