Card

Fleet Swallower

Mana cost {5}{U}{U}

Whenever this creature attacks, target player mills half their library, rounded up.

Part of 5 known combos, 3 of them two-card. 2 official rulings.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Bruvac the Grandiloquent, Riverchurn Monument, Keening Stone
B2 Core — Not in the same deck as its two-card combo partners: Bruvac the Grandiloquent, Riverchurn Monument, Keening Stone
B3 Upgraded — Not in the same deck as its early two-card combo partners: Bruvac the Grandiloquent, Riverchurn Monument, Keening Stone
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Two-card combo with Bruvac the Grandiloquent, Riverchurn Monument, Keening Stone (early with Bruvac the Grandiloquent, Riverchurn Monument, Keening Stone)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

5 known combos, 3 of them two-card.

2017-09-29 · Wizards of the Coast
Fleet Swallower’s triggered ability can target any player, not just the player it’s attacking.
2017-09-29 · Wizards of the Coast
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.

Rulings: Scryfall · Wizards of the Coast