"Captain, I think that island is following us."

Fleet Swallower

Whenever Fleet Swallower attacks, target player mills half their library, rounded up.

Secret Lair Drop

Ixalan

Whenever Fleet Swallower attacks, target player puts the top half of his or her library, rounded up, into his or her graveyard.

Ixalan Promos

Fleet Swallower’s triggered ability can target any player, not just the player it’s attacking.

If two Fleet Swallowers attack, each trigger determines the top half of that player’s library separately. For example, if the target player has fifteen cards in their library, that player puts the top eight cards into the graveyard, then puts the top four cards into the graveyard.