What is this true damage I keep seeing?

I honestly can't tell what it is, but I keep seeing champions apply true damage on multiple attacks. It's not red buff, because I've seen Singed poison and Anivia R apply it as well, and it shouldn't be cheap shot because it only applies to one hit every 4 seconds, but I see it stack up 3 times within a second or two. Something like 20-40-60. I don't see any rune aside from cheap shot that deals true damage.