Activate Viscera Seer by sacrificing Iridescent Drake.
When Iridescent Drake dies, Unhallowed Pact triggers, returning Iridescent Drake from your graveyard to the battlefield.
When Iridescent Drake enters the battlefield, it triggers, returning Unhallowed Pact from your graveyard to the battlefield attached to Iridescent Drake.
Resolve the Viscera Seer ability from step 1, causing you to scry 1.
Repeat.
This elegant loop relies on a self-sustaining cycle of sacrifice and resurrection: you use Viscera Seer to sacrifice Iridescent Drake, which triggers Unhallowed Pact to bring the Drake right back from the graveyard. As the Drake re-enters the battlefield, its own ability triggers to pull Unhallowed Pact right along with it, reattaching the Aura and priming the loop to run all over again. By repeating this sequence at will, you can churn through your entire library with infinite scry triggers while generating an endless storm of creature deaths and enters-the-battlefield events.