When does Thorncaster Sliver's ability resolve?
Ability, before blockers are even declared.



The ability triggers when the Sliver attacks, which means when it's declared as an attacker.

The ability gets placed on the stack the next time a player would get priority [CR 112.3c], which is still within the Declare Attackers step. That means the ability resolves and causes damage to be dealt in that step too.

My earlier answer covers a very similar scenario in detail.