You can not launch Shillelagh on a weapon with several damage dice
We know that when the game uses the phrase "damage die," it refers to a only die. This is evidenced by the fact that having a brutal criticism on a 2d6 weapon only adds 1d6 to the damage.
Critical Critical States:
[Y] you can throw an extra weapon damage die when determining additional damage for a critical hit …
It means that the Shillelagh the spell only replaces the damage of the weapon and so on if a weapon is damaged of this spell can not affect it.
This is somewhere similar to the state of Savage Attacker's exploit which states:
Once per turn when you get damage for a melee weapon attackyou can re-roll the weapon damage and use either the total.
At first, it seems that it would work with unarmed strikes because they count as melee attacks; However, the Sage Advice Compendium stated the following:
Q. Does the Savage Attacker exploit work with unarmed strikes?
A. No. Savage Attacker relies on damage dice from a weapon, and a strike with your bare hands is not a weapon.
Although the feat seems to be able to work with unarmed strikes, the last, more specific, weapon damage clause clause prevents unarmed strikes from functioning because they are not a weapon with a weapon. damage stream.
Likewise Shillelagh The spell apparently works for any club or staff member, but the last, more specific clause states that it modifies weapon damage. die Prevents weapons with damage dice from being affected.
the Shillelagh the spell will not work on a 2d4 club.