It grants the ability to "threaten" a Captain/Warchief, resulting in a high chance of dropping epic runes, but also increasing the target's power.
Talion issues a Death Threat through domination of a Soldier or Captain, which spares the Uruk being interrogated (it is sent away as a messenger), and warns a Captain or Warchief of Talion's intent to kill them, increasing the chances of them dropping an Epic Rune upon death. To balance this out, issuing a threat also increases their Power level significantly, and grants them extra Bodyguards, stacking with a possible "Gang Leader" Strength, making the Uruk more difficult to kill.
Within the Nemesis System's terms, the target of the Death Threat will immediately gain +5 Power (which may also cause the removal of Weaknesses and gaining of Strengths), and will always be followed closely by a large group of Uruk (usually Warriors, Archers and Hunters but may also include a large number of Berserkers and Defenders, depending on the player's progression in the game), sometimes regardless of their own class.
This ability can be useful for quick collection of epic runes, though one must take care in regards to the increased power level of the target.
If seeking a level 25 rune, then using this ability is highly inadvisable, as it will more likely guarantee an epic rune than a leveled rune. However, if a player has acquired all epic runes, then the only runes that can be further acquired by normal means are leveled runes.