Attempt to draw a card. 
The draw will be replaced by Chains of Mephistopheles, instead causing you to discard Grave-Shell Scarab. Then, the draw will be replaced by Grave-Shell Scarab's dredge ability, causing you to mill cards and return Grave-Shell Scarab from your graveyard to your hand.
Dredger's Insight's first ability triggers, causing you to gain 1 life.
Lich's Mastery triggers, attempting to draw a card.
Repeat from step 2.

Infinite self-mill

Near-infinite lifegain

Near-infinite lifegain triggers

Near-infinite self-discard triggers

Chains of Mephistopheles

Dredger's Insight

Lich's Mastery

Grave-Shell Scarab

Put a +1/+1 counter on Benthic Biomancer.
Benthic Biomancer triggers, causing you to mill a card and return Grave-Shell Scarab from your graveyard to your hand, and then causing you to discard Grave-Shell Scarab.
Thran Vigil triggers, putting a +1/+1 counter on Benthic Biomancer.
Repeat from step 2.

Thran Vigil

Benthic Biomancer

Activate Grave-Shell Scarab's cycling ability by paying {0} and discarding it, causing you to mill a number of cards and return Grave-Shell Scarab from your graveyard to your hand due to its dredge ability.
Repeat.

Rhet-Tomb Mystic

New Perspectives

Combos with "Grave-Shell Scarab"