Unstoppable Slasher does not have a counter on it.
Steps
Activate Altar of Dementia by sacrificing Unstoppable Slasher.
When Unstoppable Slasher dies, its second ability triggers, returning it from your graveyard to the battlefield with zero stun counters due to Solemnity.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to Unstoppable Slasher's power.