How does one transition to a dark welder?

Okay so I am about 2/3 of the way thru onyx storm, and I keep waiting for it to be explained but it hasn’t super clearly (or maybe it has)

Iam at the scene where Ridoc finds out about Xadeb and R and Vi are fighting.

Exactly how did Xaden start to turn venin? (Obv I know something happened between him and the sage he defeated at the end of iron flame) Can a non-venin just being channeling from the ground and that’s how it starts? That’s how I have understood it but dunno if that’s right

I would google but I’m too scared of spoilers haha

Thanks!