What it does
Compound Eyes multiplies the accuracy of every move the holder uses by 1.3. On paper that sounds minor; the actual impact depends entirely on which moves you're using.
The number that matters: Sleep Powder has base 75% accuracy. After Compound Eyes, it's 97.5%. In a format where a sleep lock on one of your opponent's two active Pokémon is a massive tempo swing, the difference between 75% and 97.5% is the difference between a coin-flip plan and a reliable one. At 75%, roughly one in four attempts fails and hands your opponent a free turn. At 97.5%, you're landing it in 39 of 40 tries.
Hurricane also benefits — 70% becomes 91%, which is meaningful if you're running a special-attack set. Bug Buzz's 100% accuracy doesn't change. Status moves like Thunder Wave aren't affected — Compound Eyes only modifies accuracy values that exist on the move itself.
Who runs it
Vivillon: 86% of Vivillon in Champions run Compound Eyes, and the reason is Sleep Powder. Vivillon's bulk is poor and its defenses won't survive a direct hit from anything strong. What it brings is a near-guaranteed sleep on command — Compound Eyes is what makes that worthwhile. Without it, Sleep Powder at 75% is too unreliable to build a gameplan around.
How to use it
The gameplan is simple enough to state: get Sleep Powder off on your opponent's bigger threat, then use the momentum to win the two-on-one. In doubles, putting one of your opponent's Pokémon to Sleep forces them to play through that turn without full resources.
Vivillon's base 110 Speed puts it ahead of most of the format without any investment. If you need to run it even faster, max Speed EVs or Choice Scarf push it further — though Choice Scarf locks you into one move, which matters if the sleep misses the 2.5% of the time.
Quiver Dance is the secondary threat: it raises Special Attack, Special Defense, and Speed by one stage each. If you sleep one opposing Pokémon and set up on the other, Vivillon becomes a real damage threat — not just a support piece. That said, Vivillon has to survive long enough to pull this off, which isn't guaranteed against aggressive openers.
Tips & strategy
97.5% is not 100%. Over a long tournament you will eventually miss at the worst possible moment. The plan should be 'this sleep is almost certain', not 'this sleep is guaranteed'. Build backup lines for the turns when Sleep Powder fails — and don't leave yourself fully dependent on it working.
Vivillon is 4× weak to Rock-type moves, because both Bug and Flying are weak to Rock. Rock Slide is a spread move used on many doubles teams, and it will almost certainly knock out Vivillon in one hit. The decision to bring Vivillon against teams that clearly carry Rock coverage is a real risk to weigh.
Against Trick Room teams, Vivillon's 110 Speed becomes a liability — it moves last under Trick Room and may take hits before Sleep Powder can fire. Evaluate whether Vivillon is worth the slot in those matchups.