If Karthus ults while his thunderlord's is up, what happens?

I don't think thunder lords would proc due to it being 3 actions but what do I know. Ludena not sure about test it.

it hits first pick

If its the third attack on said person I don't see why it wouldn't proc

They all get hit and sent to hell screaming on fire melting

thunderlords can definitely hurt multiple champions, but idk if karthus ult fully stacks thunderlords or not

it could either count as 5 hits, or 1 for thunderlords. hard to say, but I know that heimers rockets will proc it on 1 cast since each individual rocket counts as a separate instance of damage. in terms of who gets hit, I believe based on what I know about ludens interactions (i read up on the question of ludens a while back) that each person gets assigned a random number, and the person who get's the first number procs ludens. Based on this I would tentatively surmise that it would count the random numbers down to the number you need to proc thunderlords, and then the champ with that number gets thunderlords. That's assuming it counts as 5 of course.

Thunderlord's Decree is 3 attacks on one champion to proc it. Luden's Echo will proc on whomever was the first pick of the enemy team. Thunderlord's will NOT proc if you hit all 5 with one ability, You would need to hit one champion two other times then instantly ult to proc it on the SAME champion you just hit 2 times. Basically, Thunderlord's wouldn't proc on Karthus ult, Luden's Echo procs on the first pick.