You have at least seven cards in hand.

Activate Greater Mossdog's cycling ability by paying and discarding it, causing you to mill a number of cards and return Greater Mossdog from your graveyard to your hand due to its dredge ability.
Repeat.

Put a +1/+1 counter on Benthic Biomancer.
Benthic Biomancer triggers, causing you to mill three cards and return Greater Mossdog from your graveyard to your hand, and then causing you to discard Greater Mossdog.
Thran Vigil triggers, putting a +1/+1 counter on Benthic Biomancer.
Repeat from step 2.

You have a way to draw a card.

Attempt to draw card, instead choosing to mill 3 cards and return Greater Mossdog from your graveyard to your hand.
If you milled a nonland card, The Wise Mothman triggers, putting a +1/+1 counter on Benthic Biomancer.
Benthic Biomancer triggers, causing you to draw a card and discard Greater Mossdog.
Repeat.

The combo fails if you do not mill a nonland card.