Attempt to draw a card, instead discarding Dakmor Salvage due to Chains of Mephistopheles, then causing you to mill two cards and return Dakmor Salvage from your graveyard to your hand due to Dakmor Salvage's dredge ability.
The Gitrog Monster's third ability triggers, causing you to discard Dakmor Salvage due to Chains of Mephistopheles, then causing you to mill two cards and return Dakmor Salvage from your graveyard to your hand due to Dakmor Salvage's dredge ability.
Repeat step 2.
This mesmerizing loop turns a routine draw attempt into an endless engine of self-mill and discard triggers by replacing draws with dredge. Whenever a land card enters your graveyard, The Gitrog Monster rewards you by forcing a draw, but Chains of Mephistopheles steps in to intercept that draw by forcing a discard instead. This replacement satisfies the trigger and immediately dumps Dakmor Salvage into the graveyard, allowing its dredge 2 ability to replace the resulting card draw by returning itself to your hand while milling two cards. Because those milled lands trigger The Gitrog Monster all over again, the process feeds itself in a brilliant cycle.