Activate Altar of Dementia by sacrificing Golgari Thug.
When Golgari Thug dies, it triggers, putting itself from your graveyard onto the top of your library.
Resolve the Altar of Dementia ability, causing you to mill cards equal to Golgari Thug's power, ultimately milling Golgari Thug.
Colossal Grave-Reaver's second ability triggers, putting Golgari Thug from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing God-Eternal Bontu.
When God-Eternal Bontu dies, it triggers, putting itself into your library third from the top.
Resolve the Altar of Dementia ability, causing you to mill cards equal to God-Eternal Bontu's power, ultimately milling God-Eternal Bontu.
Colossal Grave-Reaver's second ability triggers, putting God-Eternal Bontu from your graveyard onto the battlefield.
Repeat.

You control another nontoken creature whose power is 1 or greater.

Activate Altar of Dementia by sacrificing another nontoken creature with power 1 or greater.
When the creature dies, Mortuary triggers, putting the creature from your graveyard onto the top of your library.
Resolve the Altar of Dementia ability, causing you to mill cards equal to the creature's power, ultimately milling the creature.
Colossal Grave-Reaver's second ability triggers, putting the creature from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing God-Eternal Rhonas.
When God-Eternal Rhonas dies, it triggers, putting itself into your library third from the top.
Resolve the Altar of Dementia ability, causing you to mill cards equal to God-Eternal Rhonas's power, ultimately milling God-Eternal Rhonas.
Colossal Grave-Reaver's second ability triggers, putting God-Eternal Rhonas from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing Ilharg.
When Ilharg dies, it triggers, putting itself into your library third from the top.
Resolve the Altar of Dementia ability, causing you to mill cards equal to Ilharg's power, ultimately milling Ilharg.
Colossal Grave-Reaver's second ability triggers, putting Ilharg from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing God-Eternal Kefnet.
When God-Eternal Kefnet dies, it triggers, putting itself into your library third from the top.
Resolve the Altar of Dementia ability, causing you to mill cards equal to God-Eternal Kefnet's power, ultimately milling God-Eternal Kefnet.
Colossal Grave-Reaver's second ability triggers, putting God-Eternal Kefnet from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing God-Eternal Oketra.
When God-Eternal Oketra dies, it triggers, putting itself into your library third from the top.
Resolve the Altar of Dementia ability, causing you to mill cards equal to God-Eternal Oketra's power, ultimately milling God-Eternal Oketra.
Colossal Grave-Reaver's second ability triggers, putting God-Eternal Oketra from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing Arashin Sovereign.
When Arashin Sovereign dies, it triggers, putting itself from your graveyard onto the top of your library.
Resolve the Altar of Dementia ability, causing you to mill cards equal to Arashin Sovereign's power, ultimately milling Arashin Sovereign.
Colossal Grave-Reaver's second ability triggers, putting Arashin Sovereign from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing Dark Revenant.
When Dark Revenant dies, it triggers, putting itself from your graveyard onto the top of your library.
Resolve the Altar of Dementia ability, causing you to mill cards equal to Dark Revenant's power, ultimately milling Dark Revenant.
Colossal Grave-Reaver's second ability triggers, putting Dark Revenant from your graveyard onto the battlefield.
Repeat.

Activate Altar of Dementia by sacrificing Avenging Angel.
When Avenging Angel dies, it triggers, putting itself from your graveyard onto the top of your library.
Resolve the Altar of Dementia ability, causing you to mill cards equal to Avenging Angel's power, ultimately milling Avenging Angel.
Colossal Grave-Reaver's second ability triggers, putting Avenging Angel from your graveyard onto the battlefield.
Repeat.