What it does
First Impression is a Bug-type physical move with +2 priority. In Champions, it has 100 base power (up from 90 in base SV — per champions_override data).
The key constraint, per Champions/Bulbapedia data: you cannot use it on the same turn you switch in. You have to wait until the following turn — and you only get one use per switch-in. Switch back out and back in, and the condition resets.
So the sequence is always: switch in, spend a turn doing something else, then land First Impression the next turn. The opponent has a full turn to see it coming.
The +2 priority means it acts before most non-priority moves and most +1 priority moves in the same turn. 100 power Bug-type hits are meaningful against Psychic-, Dark-, and Grass-type targets.
Who runs it
Ariados runs it on 10% of its Champions sets. With 90 base Attack, Ariados is not a top-tier attacker, but a +2 priority 100-power hit against Psychic and Dark types creates real pressure. The bigger value is making opponents account for the threat — when Ariados is on the board, opponents know a first-strike hit is coming next turn and have to factor that into their decisions.
How to use it
The rotation is predictable: switch in, spend the first turn on String Shot, Toxic Thread, or Protect, then land First Impression the following turn.
Using Protect on the switch-in turn is a common pairing — you stay safe for a turn, then get the priority hit off. The opponent knows the hit is coming but has to choose whether to switch, protect themselves, or just take it.
Switching out and back in resets the condition. You can use this strategically if the situation demands a second First Impression, at the cost of burning two turns and a switch.
Tips & strategy
For Ariados specifically, +2 priority is almost the only way it threatens offensive pressure. 40 base Speed means it moves last against basically everything. First Impression is the one turn where it gets to act first — and the one-turn delay means opponents at high level will anticipate it, potentially switching in a resistant target (Fire or Flying types resist Bug, and Ariados's Bug/Poison typing creates matchup problems against those types too).
Best targets: Psychic types, then Dark types. Fire- and Flying-type opponents absorb First Impression comfortably — check the opponent's roster before relying on it.