Advocates are the Scottish equivalent of barristers - specialised lawyers who can appear in the highest courts. During training one of the things they have to do is act as an unpaid assistant for a senior junior (but not a senior!) advocate for a year or so - during this time they are known as Advocate's Devils.

The process is known as devilling but, you might expect, this means different things in different parts of the UK:

https://en.wikipedia.org/wiki/Devilling#Scotland

Thanks for explaining! I only knew about the term "Devil's Advocate", I didn't realize the transposed term is a thing too.