Hi!
I have tried to explain the concept and algorithm for building the Dominator Tree of a directed graph with respect to a source vertex in this article :

https://tanujkhattar.wordpress.com/2016/01/11/dominator-tree-of-a-directed-graph/

The algorithm was published by Robert Tarjan in his original research paper here. There are not much resources on the topic, except the original research paper , on the internet. The topic is fairly advanced and involves a lot of theory.

Kindly read and give your feedbacks !

Happy Coding :)