Can it block stuff that applies on hit effects like Ezreal's mystic shot? What about stuff like Twisted Fate's gold card and Teemo's poison shot? Do you get a benefit from attacking if you have a wit's end?
Pantheon's passive will block champion basic attacks and turret shots, and any champion ability that applies on-hit effects. This includes things like Mystic Shot, and Gangplank's Parrrley.
However, any on-hit effects of the attack still apply. This means the physical damage of Toxic Shot will be blocked, but you will still be affected by the poison DoT.
In the case of Wit's End, pantheon would suffer the bonus magic damage.