Cast Doomsday Excruciator by paying {B}{B}{B}{B}{B}{B}.
When Doomsday Excruciator enters, it triggers, causing each player to exile all but the bottom six cards of their library.
Cast One Ring to Rule Them All by paying {2}{B}{B}.
When One Ring to Rule Them All enters, its first chapter ability triggers, causing Doomsday Excruciator to become your Ring-bearer and causing each opponent to mill the rest of their library.

Infinite mill

Near-infinite self-mill

Doomsday Excruciator

One Ring to Rule Them All

Cast Body of Research by paying {G}{G}{G}{U}{U}{U}, creating a 0/0 Fractal creature token, then putting a number of +1/+1 counters on it equal to your deck size.
Cast One Ring to Rule Them All by paying {2}{B}{B}, which enters the battlefield with a lore counter on it.
One Ring to Rule Them All's first chapter ability triggers, causing the Ring to tempt you, then causing the Fractal to become your Ring-bearer, then causing each player to mill cards equal to the Fractal's power.

Combos with "One Ring to Rule Them All"