You could make it part of his Duelist fantasy and passive, like so:
##Challenge:
(Your third basic attack after challenging them.)
Tohob2 votesidk about giving him a second 3-hit passive not tied to his 3-hit passive that already exists, maybe something like 
's crits where all attacks on challenged targets will crit for an amount that scales with his crit chance. in general something that interacts with the challenged target system would be cool tho.
Illâoi Evê Siôn2 votesAnd normal crits makes his Armor/MR debuff more powerful? It's be stealing from Ashe, but I could see it working.
Weebgar1 votesNot more third hit stuff. We need a unique mechanic.