If you have a good competitive moveset for Yamask, post an answer below and upvote the best ones.
Be sure to include full set details in your post, e.g. items, abilities, natures and EVs. Some explanation, including the intended game mode for your set, is also appreciated. Access the full list of guidelines here.
Yamask Pokédex and learnset for reference.
Yamask @ Eviolite Ability: Mummy Modest Nature EVs: 252 HP / 100 Def / 120 SpD / 36 SpA - Calm Mind - Toxic / Toxic Spikes / Will-O-Wisp / Destiny Bond / Rest - Shadow Ball - Psychic / Protect / Block
Hope this helps.
Gen 8 LC (SwSh)
Yamask-Galar @ Eviolite Ability: Wandering Spirit EVs: 76 Atk / 180 Def / 252 SpD Impish Nature - Toxic Spikes - Will-O-Wisp - Haze - Earthquake
Yamask-Galar @ Eviolite Ability: Wandering Spirit Level: 5 EVs: 76 Def / 196 SpA / 236 SpD Modest Nature IVs: 0 Atk - Will-O-Wisp - Calm Mind - Earth Power - Shadow Ball