Does the Thorncaster Sliver's ability damage have to be assigned before or after blockers are declared?

Question

If I have 5 slivers and one of them is a Thorncaster, will I need to assign the "1 damage to target creature or player" before or after my opponent declares blockers?

My guess is that it is as I attack, but I would like it to be after blockers are assigned.

Answer 1

As with most abilities, you choose the targets as part of placing the ability on the stack. The damage is assigned and resolves before any blockers are declared.

I will cite ikegami's answer from another question related to Thorncaster Sliver.

https://boardgames.stackexchange.com/a/12644/3128

Here's what happens:

1. You attack with five slivers. Five instances of Thorncaster Sliver's ability triggers
2. You place the abilities on the stack in the order of your choice: (Best order used)
   1. You place PS#1's ability on the stack targeting the 2/2
   2. You place PS#2's ability on the stack targeting the 2/2
   3. You place TS's ability on the stack targeting the Blood Artist
   4. You place MS#1's ability on the stack targeting your opponent
   5. You place MS#2's ability on the stack targeting your opponent
3. MS#2's ability resolves:
   1. Deal one damage to your opponent.
4. MS#1's ability resolves:
   1. Deal one damage to your opponent.
5. TS's ability resolves:
   1. Deal one damage to the Blood Artist
6. The Blood Artist dies. Blood Artist's ability triggers.
7. Your opponent places Blood Artist's ability on the stack targeting you.
8. Blood Artist's ability resolves:
   1. You lose one life.
   2. Your opponent gains one life.
9. PS#2's ability resolves:
   1. Deal one damage to the 2/2.
10. PS#2's ability resolves:
    1. Deal one damage to the 2/2.
11. The 2/2 dies.
12. [Move on the Declare Blockers]

The emphasis is mine.

Answer 2

Before blockers are declared.

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First of all, it's not combat damage, so it's not assigned. The ability triggers, the ability is placed on the stack, targets are chosen, the ability resolves, and the Sliver deals damage.

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.