Cheapest priced copy (Ixalan): $2.49 at Card Kingdom, near mint, their price list as of .
Fleet Swallower 
Creature — Fish · 6/6 · Rare · Ixalan · #57
Whenever this creature attacks, target player mills half their library, rounded up.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $2.49 to $11.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ixalan Promos | 57s | Rare | — | $6.49 |
| Secret Lair Drop | 316 | Rare | $11.99 | — |
| Ixalan | 57 | Rare | $2.49 | $3.49 |
2 rulings from Wizards of the Coast
- 29 September 2017Fleet Swallower’s triggered ability can target any player, not just the player it’s attacking.
- 29 September 2017If 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.