Unstoppable Slasher does not have a counter on it.
Steps
Activate Viscera Seer 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 Viscera Seer ability, causing you to scry 1.