Deathbloom Ritualist does not have summoning sickness.
You have at least three creature cards in your graveyard.
({2} magic symbol) (at most) available.
Steps
Activate Altar of Dementia by sacrificing Salvager of Secrets, causing target player to mill cards equal to Salvager of Secrets's power.
Activate Deathbloom Ritualist by tapping it, adding at least ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) .
Cast Twilight's Call by paying ({4} magic symbol) ({B} magic symbol) ({B} magic symbol) , causing each player to return all creature cards from their graveyard to the battlefield.
Salvager of Secrets and each creature enters the battlefield, triggering Intruder Alarm that many times and Salvager of Secrets once.
Holding priority, activate Altar of Dementia by sacrificing another nontoken creature, causing target player to mill cards equal to the sacrificed creature's power.
Resolve the Salvager of Secrets trigger, causing you to return Twilight's Call from your graveyard to your hand.
Resolve the first Intruder Alarm trigger, untapping all creatures.
Activate Deathbloom Ritualist by tapping it, adding at least one mana of any color.
Repeat from step 7 for each remaining trigger.
Activate Altar of Dementia by sacrificing another nontoken creature, causing target player to mill cards equal to the sacrificed creature's power.
Repeat step 10 until you have at least six creature cards in your graveyard.