Is there even a lategame assassin?
It seems like all assassins are focused on early to midgame. Some start off really strong like Talon and Zed, but slowly fall off by mid-lategame. Other are a bit weaker early, but if they get themselves fed they can snowball really strong in the midgame, like Leblanc and Kassadin.
But it feels like every assassins are just weak in the lategame. Adcs can just go AFK and their auto-attacks can kill assassins faster than they get bursted down. Not without counting all the peel from their team and support. And even if you happen to kill their adc, you probably either died in the process, or are forced to back afterwards because you are low hp and/or you don't have ult and are useless.

used to be a late game assassin. He could go a bit of crit and a bit of pen to deal tons of AoE while also nuking one specific person from 100 to 0. But then, Riot took away the legit Last Whisper, and then took away the crit on Youmuu's, and then reworked his whole kit so that every spell aside from his passive and Duskblade auto barely tickles anymore.
and I think
, though it depends heavily on snowball for all three.