Deathbloom Ritualist does not have summoning sickness.
You have at least three creature cards in your graveyard.
Mana Needed
({2} magic symbol) (at most) available.
Steps
Activate
Altar of Dementia by sacrificing
Mnemonic Wall, causing target player to mill cards equal to
Mnemonic Wall'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.
Mnemonic Wall and each creature enters the battlefield, triggering
Intruder Alarm that many times and
Mnemonic Wall 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
Mnemonic Wall 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.