When Felidar Guardian enters the battlefield, it triggers, targeting Vesperlark.
Holding priority, activate Viscera Seer by sacrificing Felidar Guardian.
Resolve the Felidar Guardian trigger, blinking Vesperlark.
When Vesperlark leaves the battlefield, it triggers, returning Felidar Guardian from your graveyard to the battlefield.
Repeat from step 2.
This loop turns a clever graveyard recursion puzzle into a self-sustaining engine, fueled by Viscera Seer's free sacrifice outlet to continuously cycle your pieces. By casting Felidar Guardian, you trigger an entry effect that targets Vesperlark, which you immediately bypass by sacrificing the Guardian to the Seer. That sacrifice forces Vesperlark to leave the battlefield, triggering its ability to pull the low-power Felidar Guardian right back out of your graveyard to restart the whole process.