Lochmere Serpent

Flash
{U}, Sacrifice an Island: Lochmere Serpent can't be blocked this turn.
{B}, Sacrifice a Swamp: You gain 1 life and draw a card.
{U}{B}: Exile five target cards from an opponent's graveyard. Return Lochmere Serpent from your graveyard to your hand. Activate only as a sorcery.

Throne of Eldraine

Throne of Eldraine Promos

Activating Lochmere Serpent's first activated ability after it has become blocked won't cause it to become unblocked.

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.

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.

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.

You can't activate Lochmere Serpent's last ability without targeting five cards in a single opponent's graveyard.