Sometimes the way to get a Warchief to appear in Middle-earth: Shadow of Mordor is to make one of his supporters scream.

Killing warchiefs and captains invulnerable to everything: http://wp.me/p1SfP0-1OD
Getting better and epic runes from captains and warchiefs: http://wp.me/p1SfP0-1Oy
All Shadow of Mordor Guides: http://wp.me/P1SfP0-1OQ

How to Make the Supporter Scream

The way that is done is that you grab the supporter and hold him for 20 seconds. Note that you will have to kill any other orcs in the area since they can interrupt the screaming by attacking you.