Sacrifice Lion's Eye Diamond producing {3}.
Activate Ambassador Laquatus's ability for 3 milling 3 cards.
Cast Lion's Eye Diamond for its escape cost with Underworld Breach, exiling the 3 cards milled by Laquatus.
Repeat the cycle until you've milled yourself out.

Infinite self-mill

Underworld Breach

Lion's Eye Diamond

Ambassador Laquatus

Cast Cabal Ritual from your graveyard for its escape cost by paying {1}{B} and exiling three other cards from your graveyard, adding {B}{B}{B}{B}{B}.
Activate Ambassador Laquatus by paying {3}, causing you to mill three cards.
Repeat.

Cabal Ritual

Cast Brightstone Ritual from your graveyard for its escape cost by paying {R} and exiling three other cards from your graveyard, adding at least {R}{R}{R}{R}.
Activate Ambassador Laquatus by paying {3}, causing you to mill three cards.
Repeat.

Combos with "Ambassador Laquatus"