Played Karthus. The match history said I had 2nd largest crit. HOW?

Karthus' Q is one of the few AP skills that can crit, all it really is, is a set increase in the damage if his Q only hits 1 target.

Yeah Karthus Q's isolation damage is coded as a critical hit.

There areI several other spells that show up as crit. Soraka Q also counts as a crit. Usually the tooltip says the spell can crit or it will crit. You can look up the wiki for the list if you're curious.

if you ever played a game with a nidalee in it she will always have the largest crit because of her cougar form q

When you see the floating damage dumber appear larger, bolder, with that little burst graphic on it; that's a crit. You'll see it on Karthus if he gets an isolated Q off.

Doesn't need to be basic attacks.