Let's suppose, for the sake of argument, this weapon exists in a campaign:
Unusual Club of Unusualness
Simple Melee Weapon (Club), rare
Weight: 2 lbs.
Damage: 2d4 bludgeoning
Properties: Light
The Shillelagh spell is explicitly described as:
[…] you can use your spellcasting ability instead of Strength for the attack and damage rolls of melee attacks using that weapon, and the weapon's damage die becomes a d8. […]
―Shillelagh, Player's Handbook, pg. 275
If the Shillelagh spell is applied to this specific weapon, does the damage become 1d8 or 2d8?
Best Answer
It depends on the actual wording of the Unusual Club
The magic items in the DMG (and elsewhere) have effects described by rule sentences, rather than simple shorthand (as is given in the example). Now, there are different ways to have a magic† club deal 2d4 damage:
Such a wording would mean with shillelagh we'd have two effects trying to replace the 1d4 damage die of the club. In that case the more specific one would apply, which should be shillelagh giving it 1d8 damage.
This wording means the club's magical effect does not interact with shillelagh and the club's damage with the spell becomes 1d8 + 1d4.
(This wording is similar to the bugbear's Brute trait.) Such a wording would mean the added die is changed when the base die is changed, so the club's damage with shillelagh becomes 2d8.
If your DM is not providing an actual rule with the weapon description and only the shorthand above, they have introduced a ambiguity into the game and only they can close it/rule on it. If you are the DM, make sure to give your custom magic items proper descriptions, such that your players can better predict the effect of their actions (here casting shillelagh on their special club).
†: I'm using the phrasing "magic weapon" not necessarily as magic for the purpose of game mechanics (such as resistance), but in the sense of a weapon that is a weapon specially awarded by the DM, with rules differentiating it from standard weapons. These are typically magical, and are for design purposes identical with actually magical weapons. Whether the Unusual Club is actually magical doesn't matter to the discussion.