Ivan did
it wan't stated explicitly who did it. could be ivan, could be alek. i'm leaning towards alek being the one who marked hen.
Actually they talk about it in the beginning thats why hyun made ivan take responsibility. But it affects all snake tribe people.
but it wasn't explicitly shown in the webtoon itself who marked Hen. Hen and Ivan just woke up the next day together without a lot of memories of the previous night, which made both of them (and the readers) thought that Ivan was the one who marked Hen. But now that the story is evolving, we can see some signs that maybe Ivan wasn't the one who marked Hen: he cannot control Hen's mark no matter how many times they had sex and Alek was able to calm Hen's down when he bit him in chap 80. Actually in chap 79 Alek said "That night when I started the whole thing, this wasn't what I had in mind", which can be seen as a hint that Alek was actually the one who marked Hen.
Yeah and alec left the bar waaay before hen and Ivan left. They also implied that they had sex with eachother. Its an after the fact start type of story.
The question is who marked Hyun?!?!?(╯°Д °)╯╧╧