Shillelagh Creature's weapon is magical and deals 
1d8
Bludgeoning damage. 
Attack rolls use the wielder's Spellcasting Ability Modifier.
Sources of Shillelagh
Notes
- Can only affect a quarterstaff or club, per the same name spell.
- Though not mentioned in the in-game description of the condition specifically, the damage dealt also adds the wielder's spellcasting ability modifier.