If you do, he will instead drop an Epic Rune rather than a Level 25 Rune. Take note that you should not cast a Death Threat on the targeted Warchief. From there, open 'Sauron's Army' screen and place a revenge mark on that Warchief and then proceed to learn of his weaknesses via intel documents, Uruk Works, or by interrogating Uruk Captains. With that criteria established, you will want to locate a Uruk Warchief and allow him to kill you enough times so that he reaches Level 20.

1 point for exploiting the Warchief's Fear/Hatred.

1 point for knowing the weaknesses of the Warchief.

1 point for performing a revenge kill on the Warchief who killed you.

2 points for the enemy being a Uruk Warchief.

20 points for the Uruk enemy being a Level 20 (1 point equivalent to Uruk's Power Level).

Additionally, the level of a dropped Rune is based off certain point criteria, such as:

These Runes are essentially weapon modifiers for your Bow, Dagger, and Sword that will buff the weapon with minor beneficial stats. Over the course of your playthrough, you will come to notice that when you kill Uruk Captains and Warchiefs, they will always drop a Rune.

Level 25 rune shadow of mordor