What it does
Toxic Thread deals no direct damage. It inflicts regular poison and lowers Speed by 2 stages simultaneously. In Champions, the Speed drop is 2 stages — adjusted up from the base game's 1 stage (per Serebii data).
Regular poison ticks for 1/16 max HP each turn and doesn't stack over time (that's Badly Poisoned). Both effects apply the moment the move hits: the target starts taking poison at end of turn, Speed drops that same turn.
Steel and Poison types are immune to poison — they won't take the tick damage. The Speed drop may still apply (Speed drops aren't blocked by type), but confirm whether Toxic Thread is worth the slot against targets you can't poison.
Who runs it
Ariados runs it on 11% of its Champions sets. Lower than String Shot (32%), but the two moves do different things — String Shot is pure Speed control, Toxic Thread adds persistent damage. Teams that need to chip down bulky Pokémon or lower the KO threshold for teammates get value from the poison in ways that pure Speed control doesn't provide.
How to use it
Best targets are durable support Pokémon: high-Defense walls, Assault Vest users, or the opposing team's anchor. Poison drains their HP every turn; the Speed drop makes it harder for them to act before your team does. In doubles, both effects together squeeze the opponent's Speed and longevity at the same time.
Same Speed limitation applies as String Shot: Ariados at 40 base Speed needs help acting before the target.
Tips & strategy
One underrated use: poison cancels out Leftovers and Sitrus Berry recovery. Both recover 1/16 HP per turn; poison costs 1/16 HP per turn. They offset exactly. The opponent's recovery item stops doing anything useful. If you suspect Leftovers, landing Toxic Thread makes that item dead weight.
Paired with String Shot: a target that takes both drops a total of 4 Speed stages and is poisoned. Under Trick Room, Ariados acts first naturally and can potentially land both moves across turns without needing to outspeed.
Steel types are immune to poison. Check the opponent's typing before committing to Toxic Thread as your primary tool against any Steel-type target.