You control a nontoken Saproling and/or creature with changeling.
Steps
Activate Altar of Dementia by sacrificing a nontoken Saproling or creature with changeling.
When the creature dies, Yedora triggers, returning the creature from your graveyard to the battlefield face down as a Forest land and as a 1/1 Saproling due to Life and Limb.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to the sacrificed creature's power.