Activate Carrion Feeder 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 Carrion Feeder ability from step 1, putting a +1/+1 counter on it.